‘Be discreet on your way up,’ said the sergeant, ‘we haven’t had to evacuate yet and we don’t want to start a panic.’

We promised we’d be good and headed for the lifts. Along the way we passed a familiar fire-engine-red VW transporter with LFB livery and Fire Investigation Unit stencilled on its side.

‘That’ll be Frank Caffrey,’ I told Lesley. Ex Para, Nightingale’s contact in the Fire Brigade and, if necessary, head of the Folly’s own Armed Response Unit. Or, depending on which end of the barrel you were standing at, its very own extra-legal death squad.

He was waiting for us when the lift opened; a solid man with a broken nose, brown hair and deceptively mild blue eyes.

‘Peter,’ he said nodding. ‘Lesley. You got here fast.’

The lobby had been turned into a staging post for the forensics techs. Caffrey said they’d caught a break because the inhabitants of the other two flats on the floor were away for the Christmas holidays.

‘Cape Town,’ said Caffrey. ‘And St Gervais Mont-Blanc. All right for some, isn’t it? Good thing, too, otherwise we’d probably have to evacuate the whole tower.’ According to Frank if you evacuate one set of families from a block all the others will want to know why they weren’t evacuated too. But if you go and evacuate everyone as a precaution then a good quarter will refuse to leave their flats on principle. Plus, if you evacuate them you become responsible for finding them a safe haven and keeping them fed and watered.

‘Shouldn’t we evacuate them anyway?’ I asked as I suited up.

‘Your boss says there’s no secondary devices,’ said Frank. ‘That’s good enough for me.’

I really wished it was good enough for me.

‘Did he ever tell you what a demon trap was?’ asked Lesley.

‘I got the impression they were like a magical landmine but he never said how they worked. It’s probably more fourth-order stuff.’

‘Oh, strictly second order, I assure you,’ said Nightingale, who was standing in the doorway watching us. ‘Any fool can make a demon trap. It’s rendering them safe that takes skill.’

He beckoned; we followed.

It was even stuffier than on our first visit and there was a strong odour of spoiling fish. ‘Is that real?’ I asked.

‘I’m afraid so,’ said Nightingale. ‘Salmon left out in the kitchen. A very bright young man estimated that it had been there since Monday evening.’

‘Which means they scarpered right after we interviewed them,’ said Lesley.

‘Quite,’ said Nightingale.