Edie shrugged. ‘Tara’s meant to be the best. But she hasn’t given us anything yet.’

Then a mid-Atlantic voice called me.

‘Max? Come and have a look at this.’

Dr Joe Stephen, a forensic psychologist from King’s College London, was at a workstation with someone else I didn’t recognise, a bald but bearded middle-aged man with a sweat patch in the shape of Australia on the back of his corduroy jacket. They were also watching the hanging. And I saw that the man with Dr Joe was not middle-aged at all. Beyond the bald head and the beard he was perhaps only thirty but there was something prematurely aged about him. His head was remarkable – so oval that it looked like a rugby ball impersonating a hard-boiled egg.

‘Murder by hanging is almost unknown, isn’t it?’ the strange young man said.

Dr Joe nodded. ‘But the unsubs – sorry, the unidentified subjects – don’t think of it as murder.’ He had an American accent softened and smoothed by half a lifetime in London. ‘They clearly believe they are carrying out the death penalty for what they consider a capital crime.’

The young man nodded thoughtfully.

‘Capital from the Latin capitalis, of course,’ he said. ‘Literally regarding the head – a reference to execution by beheading.’

‘Max,’ Dr Joe said. ‘This is Professor Adrian Hitchens. He lectures in history at King’s College.’

I held out my hand but Professor Hitchens ignored it. He was looking at the frozen image on the screen before him, the last frame of this latest online execution – a glimpse of the worn, ruined brickwork of the kill site.

I took my hand away.

Perhaps he was thinking very deeply about where the kill site could be. Or perhaps he thought I was the janitor.

But my feelings were not too hurt. The Met are always wheeling in these experts for a bit of specialist advice. Some of them – like our resident psychologist Dr Joe – stick around for years. But most of them are wheeled straight out again when they prove to be no help with our enquiries. There was a very good chance that I would never see Professor Hitchens again.

Or the woman with the swinging hair.

The history man jabbed a fat finger at the screen. It was stained yellow with nicotine.

‘The building looks late Victorian,’ he said, more to himself than Dr Joe or me. ‘I’m guessing some kind of public works.’ He nodded at the dank white walls, stained green and yellow with the rot of a hundred years. ‘A madhouse? A prison? Yes, almost certainly late Victorian.’

DCS Swire and DCI Whitestone joined us.

‘Hitch,’ the Chief Super said to the history man, as if they were old buddies. ‘I understand DC Wolfe here has a theory about where the first body was dumped.’

Whitestone nodded encouragement at me. ‘You thought it could be significant that the body was left in Hyde Park, right, Max?’