As a member of the 4th Royal Sussex Regiment, he went to France as part of the British Expeditionary Force, which was positioned alongside French forces on the Belgian border.
Everyone expected Germany to invade France from the east, but within six weeks, Hitler’s Blitzkrieg – lightning war – had carried his forces through Holland and Belgium and the invasion came instead from the north.
Out-flanked and surprised, allied troops fell back to the Channel ports and an escape plan was organised using 900 ships and boats, many of them personally owned. Here is Mr Jenner’s story as told to Middy reporter Dianne Jones in 2005.
“When we arrived on the beach it was bedlam. All kinds of regiments were there, English, French, Belgian. The Germans were bombing the dock at Dunkirk and you could see the plumes of black smoke.
“I thought I stood more chance getting to a rowing boat from the beach. I could imagine chaps packed like sardines down below the deck of a destroyer at the dock being bombed and killed.
“The only time I prayed without going to church was on that beach. I was 19 at the time and there were so many killed.
“German bombers returning from Britain were strafing the beach with machine-gun fire. You had to run like hell to get to the sand dunes under cover.
“There were two people hit near me. They were lying semi-conscious on the beach and I dragged their bodies up to the dunes.
“You could hear shelling from the German advance a mile or so away. I volunteered twice to go back into Dunkirk to put up defences to block the roads. We put up prams and dustbins, wheelbarrows, big stones – anything we could find. In between this we were wading out to sea up to our chests to look for boats.
“Unless you waded out you couldn’t see the boats. It was so shallow, the big boats couldn’t come in. It was like a mill pond, just like a mirror.
“All the bodies in the sea were just rocking to and fro. As you waded out there were bodies either side of you. When you got up to chest height you couldn’t see the seabed. You could tread on someone’s face, trip over them. You didn’t know if they had drowned wading out or if they had been blown up in the dock and floated down.
“I waded out four or five times. My wet clothes were covered in salt, the sand stuck to them. As they dried they felt horrible.
“On the beach I had half a tin of bully beef and my water bottle. It was filled with rum from two bottles of rum I’d found in a cafe in Belgium.
“By the second day, the beef was going off but that was all I had. One sip of rum at a time kept me going.
“I waded out with the others on the fourth night but in the morning I couldn’t find them. I looked round and I was the only one there, not a soul. I could see from one end of the beach to the other.
“I walked up and down the beach listening to the German advance. I had two rifles on one shoulder and a Bren gun on the other, 50 rounds of ammunition and a grenade in each pocket.
“I waded into the sea again. I must have spent an hour in the water before I saw a sailor in a white hat.
“He said: ‘Come on, boy’. Great big chap he was. He took my rifles and my bren gun and chucked them in the sea and he hauled me over the side of the rowing boat like a bucket.
“There were two others. When I looked back there wasn’t a soul from one end of the beach to the other, just the dead bodies. I think I must have been the last person to get away.
“The next thing I knew I was down in the hold of a paddle steamer, The Medway Queen. It was about 5.30 in the morning. I was absolutely worn out.
“To this day I don’t know how I got off the boat at Ramsgate.
“I’ve never told many people about what happened. I blanked it all out. I was 19 and had to grow up over night. Unless you go through it you can’t believe it.”
In 2004, the BBC broadcast a three-part documentary drama called Dunkirk. It starred Timothy Dalton and Benedict Cumberbatch.
Mr Jenner said of the programme: “People in the television series were laughing too much for me. I never laughed the whole time I was on the beach.
“The actors’ helmets were quite new. Their uniforms were too clean. My helmet had dents all over.
“The film had white rowing boats. There were no white boats – they would have been blown out of the water.”
Mr Jenner went on to serve in the Middle East. He was promoted to sergeant and miraculously made it to the end of the war despite being wounded five times.
He was demobbed in 1946 and settled back in Haywards Heath to raise a family.
Mr Jenner told the Middy: “I keep myself to myself. I have medals but I don’t value them.
“Unless you were there you can’t understand it and to talk about it seems like showing off.” Organising trains to transport rescued Dunkirk troops In 2005, the Mid Sussex Times published an article about Bernard Holden’s underground railway role in the Dunkirk evacuation.
The story came to light when he published his book – Let’s Make Steam – about his life on the railways.
The 95-year-old, from Ditchling, was known for his devotion to the Bluebell Railway at Sheffield Park, where Dame Vera Lynn helped him to launch the book.
In the grim days of Dunkirk, as the advancing German forces threatened to wipe out the retreating British Expeditionary Force, Bernard found himself at the crossroads of one of the most remarkable events in the history of warfare.
Hundreds of thousands of troops were pouring off the armada of ships and small boats at Dover and then being sent to other parts of the UK under Operation Dynamo.
But the trains were also playing a key role in getting the troops away from the congested coast to be re-grouped.
And every day, Bernard left his home and family in Leylands Road, Burgess Hill, and headed for an underground control centre in Redhill.
There Bernard and other controllers plotted the movement of nearly 400 trains gathered from all over the country as they were sent onwards via Woking and Reading.
The trains would be sent empty to the Kent ports and return packed with troops who, despite the inglorious retreat, were given a great welcome by the public.
Bernard, who referred to the Dunkirk retreat in his book, said: “We had to concentrate very hard on it. It was tremendous. Near the end we were still pushing empty down trains to the coast.
“But instead of coming back full they were coming back empty. It was then we knew for sure that the evacuation was all finished.”
One of the Bluebell’s former officers, the late Captain Peter Hanisty, was remembered by Bernard as being one of the key figures on Dunkirk beach, organising the evacuation.
And he also recalled one of the most moving events of his life – hearing the young Vera Lynn sing of home to 1,000 troops in India.
A few facts and figures about the evacuation of Dunkirk.
1: Between May 27 and June 4 1940, 338,226 troops were evacuated from the beaches of Dunkirk.
2. Some 98,780 men were lifted from the beaches; 239,446 from the harbour and the mole – a wooden breakwater protecting the harbour – at Dunkirk.
3: Called Operation Dynamo, the evacuation involved 933 ships, of which 236 were lost and 61 put out of action
4. The number of little boats that sailed on their own initiative will never be known – but they are known to have counted into the hundreds.
5: Many French, Belgian, Dutch and Norwegian ships took part in the operation alongside the ships of the Royal Navy.
6. The British Expeditionary Force left the following equipment behind in France, much of it to be recycled by the German Army -
76,097 tons of ammunition
416,940 tons of stores
7: Some 68,111 men of the British Expeditionary Force were captured or killed during Blitzkrieg, retreat and evacuation.
8. Of the French troops, 40,000 were taken into captivity when Dunkirk fell.
9: 126 merchant seamen died during the evacuation. 10. Winston Churchill had been Prime Minister for only 16 days when the evacuation of Dunkirk began.
11: The threat of invasion was so real that on May 29, Churchill proposed laying gas along the beaches of the south coast.
12. 1,000 Dunkirk citizens died during air raids on May 27.
13: In the early hours of May 29, the destroyer Wakeful was torpedoed and sank in 15 seconds with the loss of 600 lives.
14. 47,081 men embarked from the mole during the devastating air raid of June 1.
15: The man in charge of the evacuation was Captain William Tennant, working under Admiral Bertram Ramsay. Tennant was designated ‘beachmaster’.
16. The evacuation would not have been a success had it not been for the air cover provided by fighter aircraft which took off from the English coast.
Sources: bbc.co.uk and britannica.com