Rush hour was almost in full flood when I got on the train, and the carriage was crowded just short of the transition between the willing suspension of personal space and packed in like sardines. I spotted some of the passengers eyeing me up as I took a position at the end of the carriage with my back to the connecting door. I was sending out mixed signals, the suit and reassuring countenance of my face going one way, the fact that I’d obviously been in a fight recently and was mixed race going the other. It’s a myth that Londoners are oblivious to one another on the tube: we’re hyper-aware of each other and are constantly revising our what-if scenarios and counter strategies. What if that suavely handsome yet ethnic young man asks me for money? Do I give or refuse? If he makes a joke do I respond, and if so will it be a shy smile or a guffaw? If he’s been hurt in a fight does he need help? If I help him will I find myself drawn into a threatening situation, or an adventure, or a wild interracial romance? Will I miss supper? If he opens his jacket and yells ‘God is great’, will I make it down the other end of the carriage in time?

All the time most of us were devising friction-free strategies to promote peace in our time, our carriage and please God at least until I get home. It’s called, by people over sixty, common courtesy, and its purpose is to stop us from killing each other. It was like vestigia: you weren’t always aware of them but you instinctively shaped your behaviour in response to the accumulation of magic around you. This is what kept ghosts going, I realised; they lived off the vestigia like LEDs off a long-life battery, powering down to ration it out. I remembered the dead space that was the vampire house in Purley. According to Nightingale, vampires were ordinary people who became ‘infected’, no one was sure how or why, and started feeding off the magic potential, including the vestigia, of their surroundings.

‘But it’s not enough to sustain a living being,’ Nightingale had said. ‘So they go hunting for more magic.’ The best source of that, according to Isaac Newton, was human beings, but you can’t steal magic from a person, or any life more complex than slime moulds, except at the point of death and even then it isn’t easy. I’d asked the obvious question – why the blood-drinking? He said that nobody knew. I asked him why hadn’t anyone done any experiments, and he gave me a strange look.

‘There were some experiments done,’ he’d said after a long pause. ‘During the war. But the results were considered unethical and the files were sealed.’