“Fat lot of help you are,” I say, grabbing the branch anyway. I swing towards the wall. A moment of scrabbling later, and then I’m over the top, tumbling down into a prickly bush on the other side.

“Argh! Bladderwracks!” I cry out, as quietly as I can.

I get up and pick thorns out of my coat, and see that a ground-floor window is slightly open – just as it must have been three nights ago.

“You could have warned me about the bush,” I whisper up to Erwin, but he’s no longer there.

Through the window, I spy some kind of pantry or larder. It’s hard to see much, but on the tiles just inside the window I spot a patch of melting snow, and somehow I just know Violet left it there.

I heave myself up onto the sill and slide through the open window.

There’s a table in front of me, and I have to bite my tongue to stop myself from crying out when I see what’s lying there. A human body! Except, not exactly. At a second glance I realize it is actually something human body shaped.

“A wetsuit?” I say aloud.

And it is. But not the kind you might go snorkelling in on holiday – this is serious kit, complete with air tanks and a helmet with head-mounted torches. There are other things there too, strange things: lengths of old rubber hosing, a saw with long jagged teeth, harpoons.

There is also a pile of cardboard targets, like the ones you get at shooting galleries. I can’t help noticing that on each target, the bullseye is riddled with holes. Whoever fired at them is a crack shot.

And is this armour? I gasp at the sight of a steel chain-mail vest, complete with sleeves, on a hanger. I touch it and feel its shiny metal links slip between my fingers like scales.

A faint noise somewhere in the house jolts me. I mustn’t waste time. Sebastian Eels could be home at any moment. I slide the window up so that it looks closed at a glance but could be opened in a hurry, and continue into the house.

In the wide entrance hall there are small drops of melted snow at the bottom of a broad wooden staircase. My heart is pounding as I set off up the stairs, trying not to creak them.

On the first floor is a long landing with a number of doors. One is slightly ajar, and there are rustling sounds from inside. I edge towards it.

“Violet!” I whisper-shout. “Violet, is that you?”

But what if it isn’t?

What if all I’m doing is letting someone else – Eels himself, perhaps, or the Boathook Man – know that I’m there?