‘We were going to use vampires during the war?’ I’d asked, and been surprised by the look of genuine hurt and anger on Nightingale’s face. ‘No,’ he’d said sharply, and then, with more moderation, ‘Not us – the Germans.’

Sometimes when someone tells you not to go somewhere it’s better not to go there.

The genii locorum, like Beverley, Oxley and the rest of the dysfunctional Thames family, were also living beings on one level, and also got their power from their surroundings. Bartholomew and Polidori both suggested that they drew sustenance from all the diverse and myriad life and magic within their domains. I was sceptical, but I was willing to accept that they lived in symbiosis with their ‘domains’, whereas vampires were clearly parasitical. What if that was mirrored by ghosts? If Thomas Wallpenny was in some way part of the vestigia he inhabited and drew power from, a symbiont, then the revenant could be a parasite, a ghost vampire. That would explain the shrunken cauliflower brains of the victims – they’d had the magic sucked out of them.

Which meant that the summoning I’d done with the calculators had achieved nothing more than to feed Henry Pyke’s appetite for magic. But I also wondered if you couldn’t attract a revenant by spilling magic around like laying a chum line for shark. By the time the train pulled into Baker Street, I was already beginning to formulate a plan.

The tube is a good place for this sort of conceptual breakthrough because, unless you’ve got something to read, there’s bugger all else to do.

This time when I arrived at Westminster Mortuary I didn’t even have to show my warrant card. The guards on the gate just waved me through. Nightingale was waiting for me in the locker room. While I was kitting out, I gave him a brief explanation of my meeting with Tyburn.

‘It’s always the children,’ said Nightingale. ‘They’re never satisfied with the status quo.’

‘How did you save the blind man?’ I asked.

‘Apparently they’re not blind,’ said Nightingale. ‘They are in fact visually impaired. A very forceful young lady pointed this out to me at some length while we were waiting at the hospital.’

‘How did you save the visually impaired man then?’

‘I wish I could take the credit,’ said Nightingale. ‘It was his guide dog. As soon the sequestration began …’

‘Sequestration?’ I asked.

Apparently this was the term Dr Walid had invented to describe what happened when a normal human being was taken over by our revenant. It’s a legal term that refers to the process by which a person’s property is seized in order to pay off debts, or because it’s considered to be the proceeds of crime. In this case the property sequestrated was the person’s body.