TDC Billy Greene was putting a photograph of Hector Welles on the whitewall that he must have downloaded from Welles’ company’s website – one of those official portraits that big corporations take of their staff, Welles smiling with shrewd, bright-eyed confidence, as though your money would be safe with him.

It sat next to the police mugshot of Mahmud Irani.

Colin Cho and a couple of his people from the Police Central e-crime Unit were hunkered down around a laptop. Dr Joe was eating a frozen yogurt as he contemplated the giant map of London that covered one wall. And the voice analyst – Tara Jones – was at a workstation, replaying the one line of dialogue from the hanging of Hector Welles, repeatedly stopping and starting the film as her black hair swung across her face and she pushed it away.

But no DCI Whitestone.

‘No word from the boss?’ I said.

Billy shook his head. ‘Should I try to contact her?’

I thought of the blinded boy in his hospital bed.

‘Leave her,’ I said.

I was acting SIO now.

‘Do you know why you have been brought to this place of execution? Do you know – do you know – do you know?’

The graph on her laptop jumped and fell in time to the words.

‘How’s it going?’ I said, but Tara Jones ignored me, and continued to pore over that solitary line of dialogue.

‘Do you know – do you know – do you know?’

Still looking sickened from his viewing of Hector Welles, our history man, Professor Hitchens, stood before the giant map of the city.

‘So the spot where Hector Welles’ body was discovered,’ Dr Joe asked him. ‘That was Tyburn?’

‘As far as we can ascertain,’ said Hitchens. ‘But it’s not quite so simple. DC Wolfe was correct – this was Tyburn. But the reason Tyburn’s exact location is disputed is because Tyburn’s Triple Tree was portable.’ Hitchens glanced at me. ‘Tyburn was certainly here – though the gallows was probably in twenty different places over the course of the centuries. But the area seems to have deep significance for the – what’s the word? – the perps.’

‘And what did they do?’ Edie said. ‘Back in the day. They just assembled their moveable gallows like an IKEA flat-pack and then strung them up?’

‘The condemned stood on the back of a horse-drawn cart,’ Hitchens said. ‘The rope was attached around their neck to the cross-beam of the Tyburn tree. The cart drove off and death was by strangulation rather than broken neck. The condemned were mostly drawn from the ranks of the poor, but Samuel Pepys saw one of his closest friends – a gentleman – hanged at Tyburn. It wasn’t quick and it wasn’t pretty. And it wasn’t meant to be.’

‘So if we want to stake out the area,’ I said, ‘what counts as Tyburn?’