An armed guard stopped the car at the gate of the compound and looked at Eaton’s papers before waving him through. He parked on the tarmac at the side of the mansion, along with various other civilian and military vehicles.

At the main door, an army sergeant told Wilde that Lieutenant-Colonel Stephens was not on the premises, but that he could be reached by telephone. Eaton called him.

‘What have you got from the E-boat captain, Stephens?’

‘Flettner? Nothing as yet. He’s in solitary and hasn’t been interrogated. Not sure why he was brought to us, but if he’s got anything to say, solitary will soften him up. Normally our next stop would be to put him in with one or two others and listen in through bugs. After a while in solitary, they sometimes can’t stop talking when they meet up with a countryman. In his case, if nothing comes of it, we’ll just ship him off to a PoW camp. No real cause for the gallows as far as I can see.’

‘Can I talk to him?’

‘Feel free, Eaton. House rules, though. No Gestapo stuff. But barking at them is not only permissible but de rigueur. Only thing they seem to understand for some reason. Probably used to being shouted at by Hitler’s chaps.’

‘Thanks, old boy.’

‘Leave a note to let me know how you get on.’

Eaton and Wilde were directed to a cell on the second floor and a guard unlocked the door.

Flettner looked remarkably relaxed considering he had lost his ship and was now in solitary confinement. He was sitting on the edge of his bed, but instantly rose and clicked his heels with a respectful bow of his head as Wilde and Eaton entered the room. No Hitler salute, Wilde noted.

Eaton waded straight in without introducing himself, addressing the man in German, his normally refined voice louder and rougher than Wilde had ever heard it. Not quite a parade ground drill-sergeant, but heading in that direction.

‘You brought a plain-clothes agent into Britain by stealth. You have been engaged in espionage, Herr Kapitänleutnant.’

‘I was obeying orders, sir.’

‘That’s no defence in British law.’

‘But I was on active service. If there was a plan to bring ashore a spy, a U-boat would have been used, would it not?’

‘Clearly not in this case. As for you being on naval duty, that doesn’t look like a Kriegsmarine captain’s uniform to me.’

He nodded at Flettner’s casual clothes. Flettner, unshaven and tousled, looked down at his thick blue pullover and civilian trousers and boots.

‘That’s because I was soaking and coated in oil. When I was brought to land I was given these clothes, for which I was grateful.’

‘I don’t believe you. You’re part of a spy ring.’

‘I deny that, sir.’

‘Denial will not save your neck.’