I thumped the carpet a safe distance from the burn mark just to get a feel for what the normal floor surface felt like – concrete decking with a layer of hard insulator on top if I was any judge. I worked the stick back towards the centre until I felt it come down on something unmistakably metallic.

I froze.

‘Find the edge,’ said Nightingale.

I forced myself to backtrack until I was tapping concrete again. Lesley marked the spot with the chalk. I worked my way around the edge – it seemed to match the circular burn on the carpet but Nightingale said that you could never take that for granted. Once we’d established that there were no trigger pads outside the burnt area Nightingale handed Lesley a Stanley knife and we watched as she cut out a square of carpet and peeled it away.

The demon trap was a disc of metal the size of a riot shield, the kind you use for snatch arrests. The metal was a dull silver and looked like stainless steel. At the centre two circles had been incised side by side. One circle was filled with a glittering sand that reminded me of what happened to microprocessors when they were exposed to magic.

‘I’m guessing that the empty one is the first component,’ I said.

‘Top marks, Peter,’ said Nightingale.

‘So the intact circle is the second component,’ I said.

‘What we call a double-boss device,’ said Nightingale.

‘What’s this scratched into the edge?’ asked Lesley.

I looked where she was pointing and saw that there were marks etched neatly around the rim of the disc. Nightingale explained that they’d often found runic inscriptions on demon traps and the theory was that in the original Viking designs the runes had been part of the enchantment.

‘Like the Daoists?’ I asked.

‘Possibly,’ said Nightingale. ‘Comparative thaumatology is a discipline still in its infancy.’

This was a familiar Nightingale joke – meaning that I was the only one currently interested in it.

‘We spent a great deal of effort having the runes translated only to find it was mostly insults – “die English scum,” that sort of thing,’ said Nightingale. ‘Sometimes the messages were more ambiguous – “this is not a moral argument” was one of my favourites and of course there was the unknown craftsman who wrote “Greetings from Ettersberg”.’

‘What did that mean?’

‘Come and put me out of my misery,’ said Nightingale. ‘Or so we interpreted it. They’d conscripted a lot of practitioners from all over Europe, many couldn’t face what they were being made to do, some suicided, some suffered from a strange illness where they just stopped eating and wasted away. Others were tougher, undertook acts of sabotage or tried to contact the outside world. It must been a desperate hope that someone would hear them.’

‘And somebody did,’ I said.

‘Yes,’ said Nightingale. ‘We did.’

I recognised the markings and they weren’t Nordic runes.