I do a mental check. Nope, don’t remember that address. I cross-reference with my own notes, finding that my memory was correct – my notes have no mention of this address; the only one we had associated with Akkerman was down near Maastricht where Huisman had claimed to be when Lucie Muller was killed. So I’m not sure why Hank scribbled it down. Could Akkerman have owned a second property, or maybe it belonged to a relative?

A few minutes on my phone tell me that, according to the land registry, the property was bought in 1979 by a Marit Berkhout. Berkhout sounds familiar, only I can’t place from where. I quickly go back through my notes and find the section on Akkerman. And there it is, buried in the background profile every major suspect has done: his mother’s maiden name is Berkhout. Now that’s what I call police work. The doorbell goes off, and I open it to two middle-aged women, one Dutch, one African. They’re both holding glossy leaflets in their hands and the love of Jesus Christ in their hearts. Luckily Kush comes running, barking his head off at them.

‘Satan,’ I growl. ‘Satan, down boy.’

Kush, bless him, stops barking and plays along almost as if he understands.

The two ladies are clearly appalled. They snatch glances at each other and one of them nervously fingers a cross dangling on a chain round her neck.

Once they’ve left my soul to eternal damnation I give him a bit of cheese from the warm fridge.

‘Sir?’

Above me branches sway back and forth, micro-glimpses of sky in between. My back’s cold from lying on the houseboat’s roof. It’s not the most comfortable thing I could be doing but the discomfort is grounding somehow. Kush’s tied up to the non-functioning chimney stack, and whilst he didn’t appreciate it to begin with he has now settled down.

‘Up here.’

A speedboat roars up the canal, making the houseboat rock just as Jansen gets to his feet on the roof.

‘I see what Vermeer means; it is quirky up here,’ he says, taking in the deckchairs and odd assortment of flowerpots, some of which actually contain plants. ‘Ready?’