First name both Sydney and Sidney are used in various documents.

In the January to March 1922 birth index, Sydney N. Symondson, mother’s maiden name Read, Hampstead district, Greater London, volume 1a, page 943

1939 England and Wales Register
150 Christchurch Avenue, Kenton, Harrow, Middlesex
1. Henry N. Symondson, born January 22 1890, married, manager towel supply company
2. Dora G. Symondson, born February 25 1893, married, unpaid domestic duties
3. Douglas V. Symondson, born January 14 1917, single, assistant storekeeper electrical equipment
4. Sydney Norman Symondson, born December 12 1921, single, junior clerk charted accountants

Regimental Number 1160231

From Dnw.co.uk
Medals
DescriptionThree: Sergeant S. N. Symondson, No. 15 (Oxford’s Own) Squadron, Royal Air Force Volunteer Reserve

1939-45 Star; Air Crew Europe Star; War Medal, together with named transmission slip and card box of issue addressed to father at Harrow, Middlesex, nearly extremely fine (3) £120-150

The War Graves Photographic Project indicates he was with 15 Squadron, Royal Air Force, died July 19, 1941.

Footnote
Sidney Norman Symondson was killed in action whilst on a bombing operation to Lille on 19 July 1941 in a Stirling, when his plane was hit by flak while trying to bomb a secondary target at Dunkerque. The port inner engine caught fire and shortly afterwards the bomber dived vertically into the ground at Killem, 2 km. S.W. of Hondschoote, France. Six members of the crew, including Symondson were killed and two others taken prisoner of war. Sergeant Symondson is buried in Dunkirk Town Cemetery, France.

According to the book Laughter in the Air by Georgina Morgan (nee Symondson) Norman was killed in his first mission over Germany.
[As it turns out it was his 4th combat mission.]

www.findagrave.com
Sergeant Sidney Norman Symondson
Birth unknown
Death July 19, 1941
Note Sergeant (W. Op./Air Gnr.) Royal Air Force Volunteer Reserve. Age unknown
Burial Dunkirk Town Cemetery, Dunkirk (Dunquerque), Nord-Pas-de-Calais Region, France
Plot 2 Row 2 Coll. grave 4-5

International Bomber Command Centre web site
https://losses.internationalbcc.co.uk/loss/227443/

UK WWII Allied Airmen Roll of Honour
Name: Sidney Norman Symondson
Rank: Sergeant
Death Date: 19 Jul 1941
Military Base: Wyton, Hunts [Near Huntingdon and St. Ives in Cambridgeshire]
Service Number: 1160231
Unit: 15 Squadron
Command: Bomber Command
Ship: Stirling I
Occupation: Wop/Ag [Weapons Operator/Air Gunner}
Casualty: Killed in action
Burial Place: Dunkirk Town Cemetery, France
Notes: Shot Down By Flak At Killem Near Hondschoote During A Raid on A Secondary Target At Dunkirk (Primary Was Lille)
[15 Squadron flew the Stirling from April 1941 to December 1943.]

The following article was researched and written and sent to me by Nicholas Symondson, a close relative of Sidney Symondson. He also provided a photo of his headstone and other documents, including his logbook, letter to the RAF from his mother about his grave, a letter his mother from the Dunkirk Municipality.

Sidney Norman Symondson 1922 – 1941 
Sidney Norman was born in March 1922, (although I have seen one RAF document stating 12th December 1921 as his birth date).
In the June 1921 census, his parents, Henry Norman and Gwendoline where living at 74 Gascony Avenue in the Kilburn/West Hampstead area of London. Probably, they were still living there when Sidney Norman was born 9 months later.
In the 1921 census, Henry Norman’s employment was listed as a ‘Checking Clerk’ for the Initial Towel Supply Co., 300 Goswell Road, EC which is in the Islington area of London.
Gwendoline’s occupation was described as ‘Home duties’.
He was a pupil at St. Marylebone Grammer School from 1934 – 1938 i.e. from 12 to 16 years of age.
Founded in 1792, St Marylebone was one of the oldest schools in London and in the 1930’s had an excellent academic reputation. To gain entry to this school, Sidney Norman must have been a bright boy academically and, according to my parents, he also excelled at a range of sports.
Sidney’s name can be found on the school’s, ‘1939 - 1945 Honours Board’ together with another 71 former students from the school who died in the 2nd World War.
What he did after leaving St Marylebone school in 1938 is unknown but, by his 18th birthday in March 1940, the 2nd World War was in progress and all men between 18 and 41 were liable to be called up to serve in the armed forces.
Sometime after his 18th birthday he was selected by the RAF for training. In 1940, the RAF operated a deferred programme of up to 6 months for new recruits. In this system, recruits were immediately enlisted in the RAFVR (Volunteer Reserve) but, stayed in civilian life until given instructions on where to report for duty.
We don’t know the exact date when Sidney was called up but, probably by mid-1940 he had been selected by the RAF as a potential wireless operator and air gunner and his training followed accordingly.
After some basic training, his first specialist training was at No.2 Signals School, RAF Yatesbury.
Established in 1938 Yatesbury was one of the main sites used by the RAF to teach new recruits the practice of wireless operation and morse code.  This was taught both on the ground and in the air using a fleet of de Havilland Dominie aircraft at the base.
On the 29th November 1940, now Sergeant Sidney Norman, qualified as a 2nd wireless operator and his training moved on to, No. 10 Bomb and Gunnery School (B&GS), RAF Dumfries.
Sidney was based at Dumfries from the 9th December 1940 to 8th February 1941 learning the art of air gunnery on Armstrong Whitley’s and Fairy Battle aircraft and at the end of the course he qualified as an air gunner.
In April 1941 Sidney moved to 20 OTU (Operation Training Unit), Lossiemouth, Scotland.
Operational Training Units (OTU’s) prepared aircrew for operations on a specific type of aircraft or, a specific role in an aircraft. Often it was the last stage of training for aircrew before being assigned to an operational unit. At Lossiemouth Sidney honed his wireless and gunnery skills on a range of aircraft including: Anson’s for wireless work, Wellington’s and Stirling’s for air gunnery and aircraft acclimatization. Training at 20 OTU Lossiemouth was completed on the 12th May 1941.
Final training took place from 11th May to 14th June at No.3 Group Training Flight, RAF Newmarket on Wellington aircraft.
His final assessment after training at RAF Newmarket dated 14th June read:-
‘a keen type who is above average as a shot. Must be kept in practice with his shooting. Range finding good’.
Sidney’s Training Log Books closed on the 14th June having completed about 9 months of intensive training.
Sidney was now assigned to 15 Squadron operating Stirling aircraft from RAF Wyton, Cambridgeshire, his active service Log Book opened with his first flight on the 2nd July.
The background to his arrival at Wyton was that, at the beginning of 1941 the RAF had devised a new strategy called ‘Circus’ operations. A ‘Circus’ operation was a daylight attack in which a large force of fighter aircraft would escort a small flight of bombers to the target area. The plan was, the bombers would act as bait to bring German fighter aircraft into the air where they could be attacked by the RAF escort fighters. The primary purpose here was not the bombing of infrastructure important to the German forces but, for the escorting fighters to engage and destroy enemy fighter planes.
All the 15 Squadron active missions flown by Sidney Norman were ‘Circus’ operations.
In all these operations Sidney flew as the front air gunner positioned in the nose of a Shorts Stirling Mk 1 aircraft. Introduced at the beginning of the 2nd World War, the Stirling was a very capable 4 engine bomber although, it had a relatively low ceiling height in flight making it very susceptible to anti-aircraft fire.
His pilot on all his flights was Squadron Leader Tim Piper.
From the 2nd - 17th July, Sidney flew with Piper in Stirling No. N6030 but, this aircraft went missing in action on the 18th July with a separate crew and Sidney moved to Stirling No. N6018 for his final fateful flight.
In July 1941, Sidney flew a total of 12 times from Wyton.
5 flights were local training/practice flights, on the 2nd 3rd, 5th 10th and 13th July.
2 flights were ‘active’ missions abandoned before reaching the target due to bad weather, on the 15th and 17th July.
4 flights were ‘active’ missions over France:
6th July - flight of 6 planes led by Sq. Ldrs. Tim Piper and Menaul to bomb Lille Steel/Engineering factory – Circus No. 35.
The Stirling’s successfully dropped their bombs on the target and returned to Wyton with just small damage from an enemy fighter plane on Flt Lt. Gilmore’s Stirling. The escorting fighters were not so lucky, on the return journey they encountered heavy opposition and lost 7 aircraft that day.
8th July – flight of 3 planes led by Sq. Ldr. Piper to bomb Lille Chemical factory and Power Station – Circus No. 40.
The 3 Stirling’s dropped their bombs but all missed the target partly due to the heavy and accurate flak. All 3 aircraft suffered some flak damage but returned to base. The escorting fighters again met stiff resistance on the return flight and lost 7 pilots.
11th July – flight of 3 planes led by Sq. Ldr. Tim Piper to Le Trait Shipyards, Rouen but, due to heavy cloud over the target they, diverted to the secondary target, Hazebrouck marshalling yards – Circus No. 43.
This was a low-key affair; the bombs were successfully dropped but there was little enemy opposition and the fighter escorts all returned to base.
19th July – flight of 3 planes to Lille Power Station led by Sq. Ldr. Tim Piper but, due to heavy cloud over the target they diverted to the secondary target, Dunkirk docks – Circus No. 51.
As they approached Dunkirk, all the planes experienced heavy flak and N6018 was hit on the port wing resulting in a fire on the wing and around one engine. As a result of this damage, the pilot could no longer control the plane, it started to descend and Piper ordered the crew to bale out.
As the plane began to fall, they were attacked by German Fighter planes from Luftwaffe Unit JG26. According to Piper’s account, they…-
‘(they)…attacked at point blank range. Their fire practically shot off the front of the aircraft…’.
Oberleutnant Christian Eickhoff flying an ME109 was credited with finishing off Stirling N6018 which crashed near Killem just south of Dunkirk. The exact site of the crash remains unknown today.
Of the 8 crew, only the pilot, T. Piper and rear gunner, G. Armstrong, were able to bale out and they survived the war as PoW’s. The remaining 6 crew: Sidney Symondson, Arnold Hipps, Jack Bushell, Leslie Orchard, Percy Eve and Dennis Jeeves all died in the crash.
Sidney Norman had been operational for just 18 days and was on only his fourth active mission when he died just 19 years and 4 months old.
A sad epilogue to this story is, when his parents, Gwen and Norman, went to visit their son’s grave in the summer of 1946, they were shocked to find Sidney didn’t have a marked grave as they expected. His place of burial was actually across 2 plots, marked with a simple, single cross recording the presence of two bodies marked, ‘Unknown Airman’ and ‘Unknown Airman’. Gwen subsequently wrote to the Air Ministry about the absence of her son’s name on the cross and an investigation followed.
Correspondence in the ‘crash file’ included a letter from an official in Dunkirk to Gwen which explained that the bodies from the crash site were removed by the occupying German forces and passed to the local French authority for burial. Orchard, Eve and Bushell were positively identified and buried in Dunkirk Municipal Cemetery, Line 2 plots 6, 7 and 8. Due to the violence of the crash, the other bodies could not be positively identified except for one possible clue, a laundry tag on the clothing of one individual.  At this time, the local authorities actually thought they had the remains of 2 bodies and these were buried in plots 4 and 5 with a simple cross reading ‘Unknown Airman’. This was the cross seen by Gwen and Norman in 1946.
After Gwen’s letter a further investigation was made, Arnold Hipps was positively identified from the laundry tag and he was buried in a separate grave. Sergeants Jeeves and Symondson, although never positively identified, they were assumed to be the remaining body parts and they were reburied together in a single plot, sharing the headstone still present today.
End of article.
[Note Killem, France is about 15 miles SE of Dunkirk and about 2 miles from the Belgium border.]

My transcription of the first part of a letter from the above mentioned pilot Tim Piper to Mr. Jeeves, father of Dennis Jeeves. Letter provided by Nicholas Symondson.

At Great Farni, Little Heath, Berkhamstead, Herts.
4th September 1945
Dear Mr. Jeeves
Your letter has just reached me after being forwarded half round England and eventually reaching my wife, who presumably held on to it for some time then forwarded it.
First may I expressly sympathy in your tragic loss. When my second pilot took a crew of his own, I chose your son to fly with ?? Because I was impressed with his enthusiasm & ??????. On the day in question reached the target without any trouble but could not bomb on account of cloud, so we turned towards home hoping to pick up a secondary target on the way back. The cloud was solid until the coast where there were slight breaks. I thought I could see Dunkirk just ahead and was turning to avoid the town, when a salvo of ‘flak’ burst and the tail and port wing were set on fire. The tail gunner reported that the fire was outside the aircraft an impossible to put out, and the elevator control had gone so that I could not keep the nose up, so I ordered the crew to bail out. This was acknowledged by all the crew and Dennis was helping to get the front gunner [Sidney Symondson] out of his turret while others were trying to open the front escape hatch. By this time we were going down in a fairly steep dive and two enemy fighters who had been hanging around out of range all the way from the target, came in after we had dropped below the formation and attacked at point blank range. Their fire practically shot off the front of the aircraft and Dennis and the others must have been killed instantly. We were now completely out of control and I could do nothing. I managed to get through the escape hatch and luckily only hit the aerial mast, although I broke my parachute harness and was only hanging on by my feet. Afterwards the Germans told me that they had found the six members of the crew in the wreckage and they were buried at Dunkirk.
It is only the sacrifice of people like your son that made victory possible, and although we can never repay them, we can thank them that there is still an England and try to make a world such that they would be proud of, perhaps that will be the most difficult battle in our history.
End of first portion of the letter