He was motivated by a strange combination of self-interest, hunger for adventure, greed, bravery and patriotism.

He was freed from prison and trained as a spy by the Germans, but he claimed he always intended to swap sides and spy for Britain.

His handlers at MI5 were not so sure. Hence his codename: Agent Zigzag.

Researching the BBC Two documentary Double Agent: The Eddie Chapman Story, I became more fascinated than ever by his extraordinary combination of qualities, both good and bad.

I wrote Agent Zigzag, a book about Chapman, in 2007, but since then a great deal of additional material about his life has emerged: we interviewed the people who knew him, gathered photographs and footage, and consulted the Zigzag files in the National Archives, the huge trove of documents recently declassified by MI5.

But by far the most important contribution to the programme came from Chapman himself.

Chapman died in 1997, but four years earlier he was interviewed by the BBC for a programme entitled The Underworld: Thieves.

Deep in the archives of the BBC we found a box containing more than five hours of videotapes from that interview, in which Chapman discussed not only his criminal past, but every aspect of his wartime career, his womanising, his sabotage, and his life after the war.

The most extraordinary discovery was just how gleefully unrepentant Chapman was about what he happily referred to as his "villainy".

The unused footage even includes a masterclass on how crack a safe.

With remarkable foresight, the makers of the earlier documentary had decided to keep the camera rolling, despite knowing that by discussing his wartime intelligence work, Chapman was violating the Official Secrets Act.

They anticipated, rightly, that the laws might one day be relaxed, allowing the footage to be used.

Chapman had been gagged during his lifetime - when he tried to serialise an account of his spying in a newspaper, a judge ordered the entire print run pulped - and probably assumed that the interview would never be shown.

This may explain why, as you will see in the programme, he is such an uninhibited interviewee, swearing, cracking jokes, and blithely admitting to all sorts of skulduggery.

MI5 started releasing the Zigzag files in 2002, which means that Chapman can tell his story at last, from beyond the grave.

Ben Macintyre in front of a photograph of Eddie Chapman

Perhaps the most moving part of Chapman's testimony comes when he describes his love affair with Dagmar Lahlum, a young Norwegian woman he met in occupied Norway - where the Germans sent him on a nine-month holiday as a reward for his successful mission to Britain.

Chapman describes how he encountered Dagmar one evening at the Ritz in Oslo, and how they fell in love.

"We had a great love match and I had the intention of going back and marrying her," Chapman says in the recovered footage. "I'd love to go and see her again."

Chapman was parachuted back into Britain, for a second time, in 1944, with a mission to report back on where Hitler's V1 rockets were landing.

He promised to come back after the war and marry Dagmar.

With his MI5 handlers, Chapman sent misleading reports that caused the Germans to shorten their range, ensuring that many of the rockets landed harmlessly in the fields of Kent.

His mission, as far as the Germans were concerned, was a complete success. The British believed he had helped to save thousands of lives.

But he never went back for Dagmar, who was tried as a Nazi collaborator and sentenced to six months in prison.

In a way, the story of Dagmar perfectly reflects the contradictory character of Britain's most extraordinary double agent: fickle, seductive and staggeringly brave.

Ben Macintyre is the presenter of Double Agent: The Eddie Chapman Story.

Comments made by writers on the BBC TV blog are their own opinions and not necessarily those of the BBC.

I'm presenting BBC Two's documentary, also called Operation Mincemeat, and if the story sounds a little James Bond to you, that is no accident. It was partly inspired by Ian Fleming, then a young officer in naval intelligence. But it was put into action by two highly eccentric intelligence officers, Charles Cholmondeley and Ewen Montagu, neither of whom had any qualms about obtaining the body of a homeless man, and then turning him into someone else entirely.

Their plan was inspired, and entirely illegal. After the war, the officials involved tried to keep the name of the dead man a secret, but then in 1996, by accident, a key document was declassified formally identifying the 'man who never was' as Glyndwr Michael, a Welshman who had killed himself with rat poison in a disused warehouse.

I doubt such a plan would be feasible today, even in wartime. Imagine the scandal if it was revealed that British agents had deliberately stolen a dead body. One of the reasons it worked so well was that the organisers were left alone to get on with it, almost without supervision. That would never happen now.

The operation required exceptionally detailed planning. For example, they had to create a fake identity card, but had real difficulty finding someone who looked like Glyndwr Michael.

He had never been photographed when he was alive, and his dead body could not be made to look anything but dead. Eventually they spotted someone in the MI5 canteen, a fellow intelligence officer who was a dead ringer for the dead man, and hauled him off to be photographed.

On the BBC History messageboards, Pete asks an interesting question about whether the Germans ever suspected the body with the top secret documents was a plant.

British intelligence scoured the Germans' intercepted wireless messages for any hint that the ruse had been rumbled, and found none at all. On the contrary, in the words of a triumphant message sent to Churchill, "Mincemeat swallowed rod, line and sinker."

The only person in the entire German High Command who had any suspicions was Josef Goebbels, the propaganda minister, who wondered in his diary whether the documents might be an elaborate hoax.

But he was far too cowardly to share his doubts with Hitler, who never doubted the authenticity of the papers - in large part because they confirmed what he already wanted to believe.

Pete also asks how much of a success the operation was in terms of moving troops to Greece to defend against an invasion that never happened. Of course, that is very difficult to quantify, since it would have to be measured in lives saved, battles unfought, and blood unspilled.

But we can certainly say this: Sicily, the real target, was left comparatively lightly defended, and the island was conquered far faster than many had feared. An entire Panzer division was moved from France to Greece, to the precise spots identified in the Mincemeat documents.

And, perhaps most importantly, the great German assault on the Eastern Front, around Kursk, was called off once the invasion of Sicily was underway.

Urnungal is right that codenames were supposed not to refer in any way to the objective, individual or operation - a rule that was broken by all sides, throughout the war. Mincemeat was no exception. They chose the name because it appealed to their rather ghoulish sense of humour.

They did, however, re-use codenames. This was partly intentional since it was hoped that if, by any chance, the Germans did come across the code name, they might assume it referred to the earlier operation, and ignore it.

And lastly from the BBC History messageboards is Ferval's mention, of the film of The Man Who Never Was. It is indeed based on reality, but only very loosely. The book of that name, by one of the principal organisers, Ewen Montagu, was written under very particular constraints. Much had to be concealed, and parts are deliberately misleading.

The film went one stage further and, in the interests of drama, invented things that never happened and people, to coin a phrase, who never were. By the time the story reached Hollywood, it was partly fantasy.

Ben Macintyre is the presenter of Operation Mincemeat.

Read more on the BBC News website: Operation Mincemeat: How a dead tramp fooled Hitler

Comments made by writers on the TV blog are their own opinions and not necessarily those of the BBC.