Normally when I do a forced entry like that, the door twists as it pivots around the lock, but this door just fell inward with a crash and a backwash of dust. When I gingerly advanced to find out why, I saw that the lock’s bolt had been cleanly sheared off level with the strike plate, with the end still inside the socket. It had already been forced – and not by me.

‘Is it safe to come down yet?’ called Guleed.

I told her to give us a minute while I had a look round. It wasn’t that I was worried about her coming in – I was suddenly more worried about Wanda the manageress doing a runner. I pulled on my evidence gloves and went inside – cautiously.

Frank Caffrey, fire investigation officer, former para and Folly liaison is very clear that when entering a room you think might be rigged, the first thing you don’t do is automatically reach out and flip the light switch.

As he points out, that’s got to be one of the cheapest and most reliable triggers an IED can have. ‘I mean, it’s even got its own power supply,’ he said. And he likes to point out that the Faceless Man may like his magical weapons, but he’s not so stupid as to rely on them alone.

You know . . . I used to be worried that they were going to assign me to undercover work in Operation Trident – I obviously didn’t know when I was well off.

It was another store room with metal frame variable-height shelving lining the walls from floor to ceiling. About half the shelf space was occupied – mostly those at waist height for easy access. There was a thin layer of dust over the shelves and the floor. It’s hard to tell with dust, but I’d spent enough time on the job with my mum to reckon a couple of months’ worth. On the shelf closest to the door was a row of standard seventy-five litre plastic storage boxes with red clip-down lids. Their contents were a collection of angular shadows visible through the semi-transparent sides. The dust around them had been disturbed and there were clear hand-prints on the edges of the lids.

Staying on the fallen door to avoid contaminating the floor any more than I had to, I carefully levered the lid off the nearest box and had a peek inside. It looked suspiciously like old hardback books with stiff cloth-covered covers. The topmost book had the rough green cover I associated with the limited editions published by Russell House Press – one of the Folly’s own publishing arms. I picked it up and flipped it open to the title page.