I turned back just in time for the till to explode. The top blew right off and a spray of burning plastic fountained out, black smoke billowed upwards and raced across the ceiling. Molly gave a delighted shriek and I ran forward with the fire extinguisher and sprayed the CO2 over the till until it went out. Nightingale shut down his sphere of flaming death and switched on a set of extractor fans I didn’t even know the lab was equipped with.

‘Why did it explode?’ he asked.

‘The rapid breakdown of the components releases a volatile gas, hydrogen or something,’ I said. ‘I only got a C in chemistry, remember. The gas mixes with air inside the casing, there’s an electrical spark, and boom. The question I need you to answer is, does doing a spell suck magic out of an object or put magic into an object?’

The answer was, of course, both.

‘You don’t normally cover this until you’ve mastered the primary forma,’ said Nightingale. Magic, as Nightingale understood it, was generated by life. A wizard could draw on his own magic, or on magic that he’d stored by enchantment, which sounded interesting but not relevant to exploding cash tills. However, life protected itself, and the more complex it was the more magic it produced, but the harder it was to draw off. ‘It’s impossible to draw on magic from another human being,’ said Nightingale. ‘Or even a dog, for that matter.’

‘The vampires,’ I said. ‘They sucked the life out of everything in the house, didn’t they?’

‘The vampires are obviously parasitical in that way but we don’t know how they do it,’ said Nightingale. ‘Nor do we know how people like your friend Beverley Brook draw power from their environment, either.’

‘The vampire house is where I first noticed the effect on the microchips,’ I said.

‘As machines become more like men,’ said Nightingale, ‘I suppose it follows that they might start producing magic of their own. I’m not sure I see how this helps us.’ I tried not to wince at the pseudoscience, and decided now was not the time to get into that.

‘In the first instance,’ I said, ‘it means we know that whatever is doing this is sucking down enormous amounts of power, and second, it gives us another thing to look for.’

Not that we were actually finding anything. In the meantime, Seawoll’s Murder Team were assigned a particularly pointless stabbing in a pub off Piccadilly Circus. I had a sniff around but there were no vestigia, and a stupid but comprehensible motive. ‘Cheating boyfriend,’ Lesley explained one night when she came round to watch a DVD. First, boy meets girl, girl sleeps with second boy, first boy stabs second boy and runs away. ‘We think he’s hiding in Walthamstow,’ she said. Many would say that was punishment enough.