Back in the nineteenth century, when London was basically a big pile of crap with some buildings floating in it, and when a cholera epidemic was raging through the city like a drunk with a Gatling gun, they had this theory about what they were dying of. Nobody had made the link yet to infected water - not until John Snow came along in the 1850s with his epidemiological version of the New Testament. So the best idea they could come up with in the meantime was the miasma: a vast cloud of bad air, the exhaled breath of a million diseased and dying people, that drifted over London and infected you if you breathed it in.

That turned out to be bollocks, and Snow saved tens of thousands of lives when he tore the handles off the Soho pumps to prove his point. In the twenty-first century most people laugh at miasmas.

Not exorcists, though. We know better than anyone that things can gather in the air, unseen, and that you can breathe them in without knowing it. Most places have emotional resonances: random echoes of the emotions felt by the people who live in those places or walk through them.

Mostly they stay at a low level because - to use a crude metaphor - the peaks and troughs don’t match up. It’s like ripples in a pond cancelling each other out as their wave fronts intersect. Occasionally, though, if a lot of people are feeling the same thing, then instead of cancelling each other out the emotions reinforce each other, etch themselves deeper and deeper into what we flippantly call reality. When that happens you can get a very strong emotional residue hanging over a particular place and enduring over time. Schools, prisons, death camps, brothels, army barracks, even churches: they have their own psychic flavour, which exorcists pick up on the same wavelength that allows us to see the dead. We’re like dogs in that way, pricking up our ears at a whistle that nobody else can hear.

The Salisbury Estate had an aura of this kind. I could feel it from a long way away, when I got off the bus at Burgess Park and started to walk north towards it.

The closest of the towers was a good half a mile off, but already there was a thickening in the air that you could almost taste. The people around me - mothers with pushchairs, mainly, along with the occasional homeless guy and truanting kid, because this was the dead waste and middle of the day - didn’t seem to notice anything wrong. They kept right on walking, didn’t even look over their shoulders at the great grey towers looming behind them. So I knew it was my tuning-fork soul, resonating on a frequency that the rest of the world was deaf to.

The miasma intensified as I got closer, but although the individual towers of the Salisbury separated themselves out in my fie byout in ld of vision the feeling didn’t attach itself to any one of them."

"The contradiction between those two impressions - the vividness of the sensation and the vagueness of its source - came as something of a surprise.

I’ve learned the hard way that the physics of the material world don’t apply to my chosen field all that much: if they did, an enraged geist wouldn’t be able to pick me up and slam me into a wall because his massless form wouldn’t allow him any leverage. And zombies wouldn’t be able to move without a working heart to oxygenate their putrefying tissues.

But it’s a reliable rule that most hauntings have a fixed physical locus, an anchor point, where something that no longer belongs in this world has somehow got stuck and failed to move on. Finding the anchor is one of the first steps in any exorcism, because it means you can apply your leverage to the point where it’s going to have the biggest impact. It’s like aiming your fire extinguisher at the base of the fire, not at the flames.

This field of buzzing emotional energy wasn’t playing by the rules. It remained diffuse, impossible to pinpoint: my psychic compass wobbled and spun, looking for a true north that seemed not to be there.

The emotional weight that the miasma carried became more and more vivid as I approached: intensified, without narrowing down. What I was tasting in the air was a tension, a restless alertness, together with a sort of shift in my vision that made everything I was seeing subtly different - as though I was seeing it through a window that had been misted with somebody else’s breath.

I walked between the first of the Salisbury’s towers as I came off Freemantle Street, passing a primary school on my left. Kids are like dogs, too, and the two hundred or so toddlers swarming around in the playground seemed unusually subdued and thoughtful. They were playing on the climbing frames and hopscotch grids, but silently and with a disconcerting solemnity.

I looked up as the shadow of the Salisbury’s eastern-most block fell across me.