The man who ordered a pint of champagne cider at 9am on Tuesday September 3, 1940, at The Rising Sun could not have been a more atypical customer of the pub in the small coastal village of Lydd in Kent. He spoke with a foreign accent, was unsure about how to use the British currency he had with him and his clothes were wet.

Small wonder then that landlady – a Mabel Cole – suspected something may not be quite right with her latest customer, so much so that she alerted two of her neighbours. They confronted the man who told them he was Dutch before announcing: “You’ve caught me, I guess, and I don’t mind what happens to me but I refuse to go back to Germany.”

The man was Karl Meier, a 23-year-old Dutch Nazi who’d landed on the Kent coast hours earlier at the start of a very short-lived career as a German spy. And he most certainly was to mind what happened next because, after a two-day trial held in secret at the Old Bailey two months later, Meier and two other spies who’d landed with him were sentenced to death under the recently-passed Treachery Act.

One minute before 9am on December 10, Meier and one of the other spies, a Belgian called Jose Waldberg, were marched from the condemned cells to the gallows on the first floor of A Wing at Pentonville Prison in north London. Not long after the clock struck nine, both men were dead.

And Meier and Waldberg were not the only hapless Nazi spies to be executed.

Almost a month after the capture of the German spies in Kent, three more were rounded up in similarly ridiculous circumstances.

This time it was in the north east of Scotland, where a German seaplane had flown them from Norway and landed them on the Moray Firth on September 30.

The plan was for the group – two men and a woman – to split up and make their separate ways to London. They’d even been provided with UK-made bicycles, stolen from the British embassy in Oslo.

But conditions on the Moray Firth were so bad it took them nearly four hours to row ashore, during which time they ditched the bicycles. Two of them, Karl Drucke and Vera Eriksen, ended up in a remote railway station at Portgordon. Their soaked and bedraggled state was enough to arouse suspicion as it was, but they made matters worse by asking the station master where they were.

Scottish station masters were fortunately as suspicious as Kent landladies.

The police were called and both were arrested. Drucke’s case was found to contain £327 in cash (the equivalent of more than £15,000 today) and a Mauser pistol.

The remaining man, a Swiss national called Werner Walti, managed to get as far as Waverley station in Edinburgh where he was arrested. Drucke and Walti were sentenced to death at the Old Bailey in June 1941 and executed alongside each other at Wandsworth prison on August 6. Unaccountably, Vera Eriksen was never put on trial. She was interned for the remainder of the war.

In all, a dozen Nazi spies who were dispatched to this country around September 1940 were captured. Some were turned into double agents and used to feed misinformation back to their German masters.The rest were tried and executed.

They were sent over as part of Operation Lena, an attempt by the Abwehr,

German military intelligence, to spy on Britain ‘s defences ahead of a planned German invasion of this country – known as Operation Sea Lion. But it is hard to imagine a more incompetent and thirdrate espionage operation. It must have occurred to British counter-espionage that if this bunch were the best the Germans could do then Britain really had very little to worry about.

The planned invasion of this country was a complex and perilous operation. Had it actually taken place then it would have been by far the most audacious seaborne invasion ever attempted. It required top class cious seabor atte intelligence to give it any chance of success. And yet the spies sent over here were poorly selected and badly-trained: few were actually German, most were German or Dutch speakers with poor English and either crooks or committed Nazis, sometimes both.

I’ve no doubt that had I come up with characters like them in one of my espionage novels my editor would have sent it back to me with the comment that they lacked credibility. In fact, the spies were so laughably poor, some historians are now beginning to wonder whether this was deliberate.

Was it an attempt by some in the German military establishment to sabotage Operation Sea Lion? And if so, what were their motives?

Could it have been treason? An attempt to undermine Hitler just as he’d completed the occupation of much of Europe? The answer, as I discovered, could be found in Hamburg, Germany’s second-largest city.

Sophienterrasse in Hamburg is what local estate agents would describe as a highly desirable location. The city website talks of the area’s “sophisticated, exquisite charm”.

And you can see why. It’s conveniently located just over two miles north of the city centre. On either side of the neatly-cobbled road are attractive and mostly white-painted houses and apartment blocks, many in the distinctive art nouveau design.

At the eastern end of the road Sophienterrasse tapers into a pedestrian-only walkway which leads down through a series of steps to an area of well-kept parkland and beyond that, the Aussenalster, the larger of Hamburg’s two man-made lakes constructed from the River Alster.

And yet, in common with so many roads in Germany, Sophienterrasse hasn’t always been Sophienterrasse. During the SecondWorldWar it was known as General-Knochenhauer-Strasse. And it was from General-Knochenhauer-Strasse that the Operation Lena spies were dispatched to Great Britain to prepare for the planned German invasion of this country in 1940.

Half way down one side of Sophienterrasse is number 14, an enormous building, built in 1936 to a design far more brutalist than art nouveau. The building remains there to this day. On top of the imposing entrance block, two giant stone Imperial eagles gaze out across the city.

The entrance block is flanked by two large wings and at the end of each wing are two blocks, reaching out as far as the road. During the Second World War, number 14 General-Knochenhauer-Strasse was the headquarters of the 10th German Army Corps. But they were not the only occupants of 14 General-Knochenhauer-Strasse. The West Wing of the building was occupied by the Abwehr, German military intelligence.

And it is from this building that Operation Lena was devised. Although Hitler’s formal instruction for “Directive number 16” – was not issued until July 16, 1940, the German armed forces had anticipated the Fuhrer’s ambitions and began planning the invasion of Britain before the end of 1939.

And very soon it became apparent this would be an enormous, even impossible undertaking. The Army High Command came up with a plan to invade across a front of some 200 miles, from Ramsgate in Kent to Weymouth, Dorset.

They planned to send across around half a million troops, plus tanks, artillery and supplies. The German navy, the Kriegsmarine, estimated this would require more than 2,000 landing craft and began requisitioning barges, tugs and other vessels from around Europe.

Grand Admiral Raeder, the head of the navy, suggested a more limited and manageable plan: a front of just 35 miles stretching from Deal to Dungeness in Kent.

The problem for the Germans was the dominance of the Royal Navy. It was too strong and, backed by the RAF in the skies, was likely to thwart any invasion before a single soldier had set foot ashore. To counter this the Luftwaffe planned an aerial campaign to eliminate the Royal Air Force, which was to become the Battle of Britain.

But as loyal as the German generals and admirals were, whichever way they looked at it, Operation Sea Lion seemed doomed. And this was a view shared by the Abwehr. From their offices in General-Knochenhauer-Strasse in Hamburg, they realised the planned invasion was not only likely to be a failure, but it could also be a failure which could lead directly to the defeat of Germany.

What was interesting about the Abwehr was how non-political it was.

It saw itself as a professional espionage organisation rather than a tool of the Nazi Party. Its head, Admiral Canaris, was never a Nazi Party member. Nor was his deputy, General Oster. Herbert Wichmann, the man responsible for recruiting that hopeless bunch of spies, training them and dispatching them over to Britain was no Nazi either.

What if he’d come to the conclusion, encouraged no doubt by Canaris (a man who would later be executed for his opposition to Hitler) that it was in the longterm interests of Germany for Sea Lion to be aborted? In fact, none of the Operation Lena spies sent back a single shred of intelligence.

We will never know for sure but, as we do know, the Battle of Britain ended in defeat for the Luftwaffe who switched to bombing operations. And on October 12, 1940, Sea Lion was shelved – the German invasion of Britain would never be a serious proposition again.

And what now of 14 General-Knochenhauer-Strasse, now Sophienterrasse?

The enormous building is still there, seemingly unchanged from when it was built in 1936. It clearly emerged unscathed from the heavy RAF bombing of Hamburg.

Giant stone Imperial eagles remain on top of the entrance section, carefully surveying the city below them. When I visited earlier this year researching my next novel, my guide and I managed to enter the building.

It’s now an upmarket residential complex, with porters and a reception area matching that of a five-star hotel.

My guide explained I was interested in looking around as this had been a German intelligence headquarters during the war.

The man behind the plus desk looked horrified. Perhaps, he suggested in a tone which didn’t require translation, we would care to leave?

Every Spy A Traitor by Alex Gerlis (Canelo, £16.99) is out now. Visit expressbookshop.com or call Express Bookshop on 020 3176 3832. Free UK P&P on orders over £25