‘Since you are fixed for certain, and can do nothing for yourself, does it matter where you wait for rescue?’ asked Tyburn. ‘And I seem substantial to myself.’

‘You might be a ghost,’ I said, studying the ditch and wondering whether I should go in head or feet first. ‘Or a sort of echo in the memory of the city.’ I really had to come up with some better terminology for this stuff.

I jumped down into the ditch. The soil was soft, sticky yellow London clay. Head first would be quicker.

‘Or we could get a boat to Southwark,’ said Tyburn. ‘Hit the stews, get steamed – meet some hot girls from Flanders. Oh come on,’ he pleaded. ‘It’ll be kicking and I’ve …’ Tyburn trailed off.

‘You’ve what?’

‘I’ve been alone – here,’ he said. ‘For a long time I think.’

Possibly since you ‘died’ in the 1850s under a tide of shit, or so your father claims.

‘Now you’re saying things that I’ve just thought of,’ I said. ‘You see why I’m suspicious.’

This is why magic is worse even than quantum physics. Because, while both spit in the eye of common sense, I’ve never yet had a Higgs boson turn up and try to have a conversation with me.

‘Did you hear that?’ asked Tyburn.

I was going to ask what when I heard it – a long wail floating over the landscape from the direction of London. I shivered.

‘What’s that?’ I asked.

The wail came again, wordless, angry, filled with rage and self-pity.

‘You know who that is,’ said Tyburn. ‘You put him there, you pinned him to the bridge.’

As an experiment I stuck my foot in the hole, which sucked at it with an unpleasantly organic movement.

The wail was fainter the third time, fading into the wind and the noise of the passers-by.

‘Sooner or later you’re going to have to set the hooknosed bastard free,’ said Tyburn.

Not any time soon, I thought.

‘I don’t want to die in a hole with my eyes closed,’ I said, and shoved my foot in up to the ankle.

‘Don’t do that,’ said Tyburn, and jumped into the ditch with me. ‘I know a better way.’

‘Really?’ I asked. ‘What’s that, then?’

‘This,’ he said, and hit me on the side of the head with the pommel of his sword.

I regretted the decision as soon as I opened my eyes to darkness, and the feel of water sloshing around my knees. It was cold – a wetsuit won’t keep you warm if you don’t move about.