The woman also took the tube one stop, and trailed Knox along Hillmarton Road. Then, as soon as she was sure where he was headed, she doubled back on herself, shaking her head as if she’d just realised she’d left something important at home, and went back down into the Underground.

The ornate turrets of HMP Holloway looked more like they belonged to a Cambridge college than the front of a prison in north London. The imposing architecture of Holloway prison, which was originally opened in 1852, was intended to inspire respect for the law. The castle-like facade was also apt as conditions inside for most of the prison’s residents were positively medieval.

Once criminal charges had been brought against the four members of the Calder Hall Ring and the case became public, Sandra Horne had been transferred from the secure holding cell on the third floor of Leconfield House to Holloway. Knox didn’t agree with the decision.

‘We can’t guarantee her safety if she isn’t in our custody,’ he argued. ‘And Holloway is hardly the leniency we promised for cooperation.’

But he’d been overruled.

‘It’s not our decision to make any more,’ Holland said. ‘But I’ve been assured she’ll be held in solitary confinement, away from any inmate who might wish to do her harm.’

It was a peculiar character trait of many prisoners that they developed a fierce patriotism for the country that had locked them up. To many of Holloway’s long-term residents, Sandra Horne’s role in passing on classified information to the KGB was as treasonous as trying to kill a member of the royal family.

Knox stepped through the stone arch entrance to the prison and into the guard’s office. The man on duty recognised him from his previous visits to Horne and, luckily for Knox, waved him through without checking if his presence had been approved.

Five minutes later, he was sitting in a spartan meeting room, waiting for Horne to be brought to him. The room was cold. Its concrete and brick floor and walls were whitewashed, its heavy door was painted gunmetal grey. There were no windows. Knox sat on one of two chairs that faced each other across a small Formica table.

Knox knew Horne might not know anything about Bianchi and Moretti, but he still had questions about the mysterious contact in Cecil Court she’d alluded to when she’d first been persuaded to talk. Knox wanted to know if there was another Soviet agent operating in the city.