Ordinarily, Lucy did not like entering any premises unannounced or without authority. But she’d been informed that a crime had occurred here, and she could see from the state of the door that some kind of violence had been done, and so, thanks to the Police and Criminal Evidence Act, which permitted her to enter property to prevent harm or injury, she was covered legally. But, in terms of announcing herself, well – as she was alone, it felt better not to on this occasion. The other option, of course, would be to call for assistance, but that wouldn’t leave her looking good if this lead turned out to be nothing.

She ventured forward cautiously, opting to explore the ground floor first, passing the foot of the stairs and staring down the corridor that led into the guts of the building.

For a second, she wondered if she’d just heard something – a slight creak of woodwork maybe. But if she had, it had been infinitesimal, and anyway she couldn’t hear anything now.

She proceeded, padding thirty yards along bare floorboards, with only peeling plaster walls to either side, before she came to an internal door, which, when she turned the knob, opened smoothly. On the other side, she found a small, very utilitarian kitchen, little more than a closet space really, with a dirty metal sink at one end, its two taps caked in lumps of hardened paint. The cupboard space underneath the sink, which was visible because there was no door on it, contained nothing but dust and mouse droppings. Close by, on a shelf, a kettle was plugged into a power point; two tins – one containing teabags and one instant coffee, sat alongside that, next to a cup of sugar, an opened bottle of milk which had turned green, and a single mug with what, on closer inspection, looked like fungus growing inside it.

All this was just about visible thanks to the dim light spilling through a single, unshaded window. This gave out into what might once have been a rectangular yard, the narrowest Lucy had ever seen – no more than a gap between buildings – and knee-deep in festering rubbish. There was another door in here, connecting to this yard, but even if it was unlocked, there was clearly no point in trying to exit the place that way.

Then she heard another creak. And this one had not been her imagination.

It was louder than the previous one – a long, low groan of aged timbers; the sort of sound you hear from old floorboards when weight is suddenly redistributed.

Quite clearly, it came from overhead.

She moved back down the corridor to the bottom of the stairs, and waited.