‘There’s probably more history where we are standing than any place in the country,’ I said. ‘For a thousand years, Tyburn was the country’s most celebrated place of execution. More than fifty thousand men, women and children were hanged here. London was always a city of execution – in the eighteenth century you couldn’t enter the city without seeing a line of gibbets – but Tyburn was always special.’

‘Dr Joe says that ritual and ceremony was important to the perps – as important as the punishment.’

I nodded. ‘It matters to them that this was where Tyburn stood. It’s important to them that their victims are hanged, and then dumped here. I’m sure they wish they could do it on the pavement outside the Odeon Marble Arch. But I’m hoping they care too much, that it’s too important to them. I’m guessing that they are so obsessed with all that symbolism that we will have our chance to nail them.’ I turned to look at our SIO. ‘You want to get Marble Arch staked out?’

‘I should have done it sooner,’ Whitestone said. ‘After the first one. It’s not a difficult place for a team of undercover officers to watch, especially in summer when there are more bodies sleeping out around Hyde Park and Marble Arch. But if they do it again, they have to come back here. And next time we’ll be waiting.’

‘And they’re going to do it again, aren’t they?’ I said.

‘I don’t see how they can stop now,’ she said.

Edie consulted her phone. ‘The Divisional Surgeon has arrived to check that Welles is really dead,’ she said. ‘I’ll escort him in.’

I looked at Whitestone.

‘And the history man is here,’ I said.

‘Let’s give him one last try,’ Whitestone said.

I walked across to the perimeter and the old-looking young man who was waiting there. He had got off his bike and placed the helmet on the pillion and was sucking on a soggy roll-up cigarette. It did not seem to be giving him much joy. Despite the motorbike, and despite the fact that it was going to be another brutally hot day, Professor Adrian Hitchens wore a two-piece corduroy suit, a shirt and tie and a V-neck jumper that had been munched by moths long gone. His head still looked remarkable to me – so egg-shaped that it was almost pointed. It glistened with heavy beads of sweat.

‘Professor Hitchens,’ I said.

‘I feel that we got off to a bad start,’ he said. ‘You and I. Your theory about Tyburn – I dismissed it out of hand. That was wrong. You were correct. And I apologise.’