He recalled one of the many briefings in Matlock – the tutorials, as Hendrie liked to call them. ‘Remember, old chap, a chain is only as strong as its weakest link. Rather a cliché, I know, but a very wise one nonetheless and one you’d be well advised to keep in mind. At any and every stage of your mission there’s bound to be one part of your cover – your story, if you like – that is not quite as robust as the other parts. Be prepared for that.’

And now – possibly too late – he decided the accreditation letter from Mortensen Machinery Parts was the flaw, Hendrie’s weakest link. It was all well and good Agent Horatio sorting matters out with his managing director, but what would happen if the Gestapo called the switchboard and they replied that they had no employee called Peter Rasmussen?

‘Kommen Sie her!’ The Gestapo officer was calling him and Prince could feel his throat tightening, convinced there was a menace to the man’s voice. As he walked back to the desk, he noticed a couple of SS officers standing in front of the exit; he was sure they’d not been there when he last looked.

But the man with the improbably thick spectacles nodded his head to indicate all was in order and handed the papers back to Peter Rasmussen. ‘Willkommen im Reich.’

Prince bowed very slightly in gratitude and muttered, ‘Vielen Dank,’ with a slightly better pronunciation than was perhaps wise. When the Gestapo officer enthusiastically shouted, ‘Heil Hitler!’ he decided the wisest course of action was to respond likewise.

They were in Kreuzberg now, and it was a five-minute walk from the Kochstrasse U-Bahn station to Askanischer Platz. ‘We ought to assume we’re being followed,’ Knudsen said. They were crossing Wilhelm Strasse, and he had his head down as he spoke. ‘As I said, getting into Germany is not so difficult. It’s easier to break into a prison than out of one, isn’t it? They will be keeping a close eye on us, certainly for the first twenty-four hours. They’ll want to know what we’re up to while we’re here. There are rumours they let in people they know are here to cause trouble just to see who they meet with and what they get up to. So we’ll do nothing suspicious. In fact, I’m making it easy for them to keep an eye on us.’