There was disagreement among the authorities as to what the true nature of a ghost was. Polidori insisted that ghosts were the detached souls of the deceased who clung to a locality. He theorised that they fed off their own spirit and would, unless this spirit was replenished through magic, eventually fade away to nothing. Richard Spruce’s The Persistence of Phantasmagoria in Yorkshire, published in 1860, broadly agreed with Polidori but added that ghosts might draw on the magic in their environment in a similar manner to a moss leaching sustenance from its rocky home. Peter Brock, writing in the 1930s, theorised that ghosts were nothing more than recordings etched into the magical fabric of their surroundings in much the same way music is recorded on a vinyl disk. Personally I figured that they were like crude copies of the dead person’s personality that were running in a degraded fashion in a kind of magical matrix where packets of ‘information’ were passed from one magic node to the next.

Since both my encounters with Nicholas had started in the portico of the Actors’ Church, that was where I began. Coppers don’t look at the world the same way other people do. You can tell a policeman by the way he looks around a room. It’s a chilly, suspicious gaze that makes him immediately recognisable to others who know what to look for. The strange thing is how fast you pick it up. I was a still a Police Community Support Officer, had only been doing it a month, when I visited my parents’ flat and realised that even if I didn’t know already that my father was an addict, I would have spotted the fact the moment I was in the door. You have to understand that my mum is a cleaning fanatic – you could eat dinner off her living-room carpet – but still all the signs were there if you knew what to look for.

It had become the same with vestigia. When I put my hand on the limestone blocks that made up the portico, the sensations, the cold, vague sense of presence, an odour in the nostrils that might be sandalwood, were the same – only now, like a copper reading a street, I had some inkling of what they meant. I also expected them to be much stronger. I tried to think back to the last time I’d touched the stones. Had the impressions been the same?

I checked to make sure nobody was watching. ‘Nicholas,’ I said to the wall. ‘Are you in there?’

I felt something through my palm, a vibration, I thought, like a distant tube train. Toby whined and scrambled backwards, claws skittering on the cobbles. Before I could take my own step backwards Nicholas’s face, white and transparent, appeared in front of me.

‘Help me,’ he said.

‘What’s wrong?’ I asked.