The two young officers had been briefed on all the talk rattling around Whitehall about angry Catholic nationalists and fervent Protestant unionists. But after several weeks of backstreet meetings and shadowing operations, Knox and Williams had concluded that for the moment talk was all it was. The Irish and the Northern Irish, like the rest of Europe, weren’t ready to take up arms again just yet. So for the rest of their deployment, Knox and Williams had spent their mornings dutifully filing their observation reports and their afternoons and evenings in the pub.

‘We had some fun,’ Williams said.

‘Yes, we did,’ Knox agreed.

He was buoyed by Williams’s positive reaction to one of their most formative experiences in MI5, and wondered what else he could help him remember from their early days in the Service.

‘What about St John Clare?’ he asked.

Williams’s face went blank for a moment while he tried to place the name, then his smile came back, even broader than before.

‘The Red Tory,’ he said, his smile growing wider. ‘The barnstormer in the top hat and tails who was secretly working for the Hungarians.’

St John Clare was a young right-wing firebrand who had seemingly come out of nowhere to be elected as MP for North Devon in 1951 when Churchill was voted back into 10 Downing Street. He was as close as Britain came to having its own Joseph McCarthy, campaigning loudly for witch hunts against suspected communists, until he suddenly and quietly stepped down and triggered a by-election in 1953.

‘Hated everyone except dyed-in-the-wool Conservatives, but was more than happy to take Budapest’s money,’ Knox said.

It was MI6 that had first uncovered the signs of Clare’s hypocrisy and potential betrayal of his nation, and it had done it entirely by accident.

Hungary had been occupied by Soviet forces since the end of the war, so most Western intelligence about the country had to be gathered clandestinely. An MI6 analyst had been comparing a new set of undercover photographs taken in the Hungarian capital with some old file images when they realised that one of the people in the background of a photo taken in 1947 of Liberty Bridge, the first of the city’s crossings over the Danube to be rebuilt after the Nazi retreat, looked remarkably like Clare. MI6 had passed their discovery on to MI5, and Knox and Williams had been sent down to Dorset during the 1952 parliamentary winter recess to ask Clare to explain himself.

‘I’ve never seen anyone crack so quickly during questioning,’ Knox said.

‘What questioning?’ Williams said. ‘We were barely through his front door before he broke down and confessed everything.’

It had taken roughly ten minutes for Clare to tell them that he’d spent the last five years taking money from a ‘representative of the Hungarian government’ he’d happened to meet during his holiday in Budapest for information he passed on about the British upper class.