There was a black-and-white picture of a smiling rabbit-faced man from the middle of the last century. The account was called @AlbertPierrepointUK. No message. Just the hashtag – #bringitback – and a link to the film.

‘It’s got just under twenty-five thousand followers,’ Edie said. ‘No – over seventy-eight thousand followers. Wait—’ She leaned back in her chair and sighed. ‘Wow, popular guy, this Albert Pierrepoint. Why is the name so familiar?’

‘Albert Pierrepoint was the most famous hangman this country ever had,’ I said. ‘He carried out more than four hundred executions, including a lot of the Nazis in Nuremberg.’

‘Metcall have had a 999,’ Billy said, putting down the phone. ‘From a woman who recognises the victim.’ He looked up at the screen and winced at the man once more locked in the final throes of agony. ‘The woman’s a Fatima Irani from Bethnal Green. The man is Mahmud Irani. Her husband.’

‘How do you spell his name?’ Whitestone said. ‘Got a DOB? Got a description of what he was wearing?’

Greene read from his notes. Then he looked up at the screen.

‘She said her husband was wearing jeans and one of those shirts with the little crocodile,’ he said, and stooped to retch into a wastepaper basket. It took him a moment to recover. ‘Sorry,’ he said.

‘Play it again,’ Whitestone said. ‘Have a drink of water, Billy. Are you looking on the PNC, Edie?’

Edie Wren was running the name of Mahmud Irani through the Police National Computer.

‘He’s been away,’ she said, meaning the man had done time. ‘Did six years of a twelve-year sentence. He was part of the Hackney grooming gang. They targeted girls as young as eleven. A lot of the girls – but not all of them – were in care. Some of the gang got life. This Mahmud Irani was found guilty of trafficking – he’s a taxi driver. He was a taxi driver. He got off relatively lightly.’

We watched him hang for the third time.

‘Maybe not that lightly,’ I said. ‘If this is connected.’

A young Chinese man appeared in the doorway of MIR-1. He was Colin Cho of PCeU – the Police Central e-crime Unit, jointly funded by the Home Office to provide a national response to the most serious crimes on the Internet.

‘We’re looking for Albert Pierrepoint,’ he told Whitestone, nodding at the big screen. ‘He – they – seem to be using exactly the same tech as terrorists, pornographers and whistle blowers. The account is running through an anonymiser designed to hide all digital footprints. But it’s not Tor or 12P. It’s something we have never seen before. The site’s under a lot of pressure – political, media, users, concerned parents – to take the film down in the name of decency, but we’ve persuaded them to leave it up there while we try to trace the sender’s IP address. Off the record, of course.’