A new wave of spies began in September. Their task was to reconnoitre Britain in preparation for the invasion to map beaches suitable for landings; fields and roads that could be used by troop-carrying gliders; tank traps and road blocks and barbed-wire obstacles.

They seem to have been badly selected, hastily mustered, inadequately trained, and poorly equipped. Typical were the four who came over on the night of 23 September: Meier, Kieboom, Pons, and Waldberg. Kieboom and Pons landed at dawn near Hythe, and were arrested by Private Tollervey of the Somerset Light Infantry, who came upon them in the sand dunes hacking away at a dirty great wurst.

Waldberg actually managed to send a signal to Hamburg: ARRIVED SAFELY. DOCUMENT DESTROYED. ENGLISH PATROL 200 METRES PROM COAST. BEACH WITH BROWN NETS AND RAILWAY SLEEPERS AT A DISTANCE OP 50 METRES. NO MINES. FEW SOLDIERS. UNFINISHED BLOCKHOUSE. NEW ROAD. WALDBERG.

Clearly he did not know where he was, nor did he even have a code name. The quality of his briefing is indicated by the fact that he knew nothing of English licencing laws: he went into a pub at nine o'clock in the morning and asked for a quart of cider.

(Godliman laughed at this, and Terry said: "Wait: it gets funnier.")

The landlord told Waldberg to come back at ten. He could spend the hour looking at the village church, he suggested. Amazingly, Waldberg was back at ten sharp, whereupon two policemen on bicycles arrested him.

("It's like a script for 'It's That Man Again'," said Godliman.)

Meier was found a few hours later. Eleven more agents were picked up over the next few weeks, most of them within hours of landing on British soil. Almost all of them were destined for the scaffold.

("Almost all?" said Godliman.

Terry said: "Yes. A couple have been handed over to our section B-l(a). I'll come back to that in a minute.")

Others landed in Eire. One was Ernst Weber-Drohl, a well-known acrobat who had two illegitimate children in Ireland. He had toured music houses there as 'The World's Strongest Man.' He was arrested by the Garde Siochana, fined three pounds, and turned over to B-1 (a).

Another was Hermann Goetz, who parachuted into Ulster instead of Eire by mistake, was robbed by the IRA, swam the Boyne in his fur underwear and eventually swallowed his suicide pill. He had a flashlight marked "Made in Dresden."

("If it's so easy to pick these bunglers up," Terry said, "why are we taking on brainy types like yourself to catch them? Two reasons. One: we've got no way of knowing how many we haven't picked up. Two: it's what we do with the ones we don't hang that matters. This is where B-l(a) comes in. But to explain that I have to go back to 1936.")

Alfred George Owens was an electrical engineer with a company that had a few government contracts. He visited Germany several times during the '30s, and voluntarily gave to the Admiralty odd bits of technical information he picked up there.

Eventually Naval Intelligence passed him on to MI6 who began to develop him as an agent. The Abwehr recruited him at about the same time, as MI6 discovered when they intercepted a letter from him to a known German cover address. Clearly he was a man totally without loyalty; he just wanted to be a spy. We called him "Snow"; the Germans called him "Johnny." In January 1939 Snow got a letter containing (1) instructions for the use of a wireless transmitter and (2) a ticket from the checkroom at Victoria Station.

He was arrested the day after war broke out, and he and his transmitter (which he had picked up, in a suitcase, when be presented the checkroom ticket) were locked up in Wandsworth Prison. He continued to communicate with Hamburg, but now all the messages were written by section B-l(a) of MI5. The Abwehr put him in touch with two more German agents in England, whom we immediately nabbed. They also gave him a code and detailed wireless procedure, all of which was invaluable.

Snow was followed by Charlie, Rainbow, Summer, Biscuit, and eventually a small army of enemy spies, all in regular contact with Canaris, an apparently trusted by him, and all totally controlled by the British counterintelligence apparatus.

At that point MI5 began dimly to glimpse an awesome and tantalising prospect: with a bit of luck, they could control and manipulate the entire German espionage network in Britain.

"Turning agents into double agents instead of hanging them has two crucial advantages," Terry wound up. "Since the enemy thinks his spies are still active, he doesn't try to replace them with others who may not get caught. And, since we are supplying the information the spies tell their controllers, we can deceive the enemy and mislead his strategists."

"It can't be that easy," said Godliman.

"Certainly not." Terry opened a window to let out the fug of cigarette and pipe smoke. "To work, the system has to be very near total. If there is any substantial number of genuine agents here, their information will contradict that of the double agents and the Abwehr will smell a rat."

"It sounds exciting," Godliman said. His pipe had gone out. Terry smiled for the first time that morning. "The people here will tell you it's hard work: long hours, high tension, frustration, but yes, of course it's exciting." He looked at his watch. "Now I want you to meet a very bright young member of my staff. Let me walk you to his offlce."

They went out of the room, up some stairs, and along several corridors. "His name is Frederick Bloggs, and he gets annoyed if you make jokes about it," Terry continued. "We pinched him from Scotland Yard-he was an inspector with Special Branch. If you need arms and legs, use him. You'll rank above him, of course, but I shouldn't make too much of that; we don't, here. I suppose I hardly need to say that to you."

They entered a small, bare room that looked out on to a blank wall. There was no carpet. A photograph of a pretty girl hung on the wall, and there was a pair of handcuffs on the hat-stand.

Terry said, "Frederick Bloggs, Percival Godliman. I'll leave you to it."

The man behind the desk was blond, stocky and short; he must have been only just tall enough to get into the police force, Godliman thought. His tie was an eyesore, but he had a pleasant, open face and an attractive grin.

His handshake was firm.

"Tell you what, Percy, I was just going to nip home for lunch," he said. "Why don't you come along? The wife makes a lovely sausage and chips." He had a broad cockney accent.

Sausage and chips was not Godliman's favourite meal, but he went along. They walked to Trafalgar Square and caught a bus to Brixton. Bloggs said, "I married a wonderful girl, but she can't cook for nuts. I have sausage and chips every day."

East London was still smoking from the previous night's air raid. They passed groups of firemen and volunteers digging through rubble, playing hoses over dying fires and clearing debris from the streets. They saw an old man carry a precious radio out of a half-ruined house.

Godliman made conversation. "So we're to catch spies together."

"We'll have a go, Perce."

Bloggs' home was a three-bedroom semi-detached house in a street of exactly similar houses. The tiny front gardens were all being used to grow vegetables. Mrs Bloggs was the pretty girl in the photograph on the office wall. She looked tired. "She drives an ambulance during the raids, don't you, love?" Bloggs said. He was proud of her. Her name was Christine.

She said. "Every morning when I come home I wonder if the house will still he here."

"Notice it's the house she's worried about, not me," Bloggs said.

Godliman picked up a medal in a presentation case from the mantelpiece. "How did you get this?"

Christine answered. "He took a shotgun off a villain who was robbing a post offlce."