Where's the rear 12V socket fuse?

I've blown a couple of fuses plugging in a dodgy appliance. Silly me.
The vehicle has the Type 2 fuse box so found the fuses for the front and boot sockets, F13 and F14 in the glove compartment OK which got the front one working again. But I can't find the fuse for the rear one.
For the Type 1 fuse box all three are listed, but for the Type 2 the book only refers to the front and boot, not the rear.

I found a reference in a forum to the fuse being in the extra fuse box in front of the battery. But the manual lists only three fuses in there and none are for a 12V socket.

Also, if it is in there, how do I open it? It doesn't seem very obvious. It's a Valeo unit and says on it "Before to remove cover, unscrew the product". But I can't even see how to unscrew it!

So, anyone know where I can find the fuse for the rear 12V socket?

Vehicle is a C4 Grand Picasso 2.0 Blue HDi Exclusive 5dr EAT6 B78, 14 plate