Hey - sorry about this, but I got the fuse boxes the wrong way around. I'll edit my original post to correct it.
The picture I took is misleading, in that it looks like the thick blue wire (which is the accessory switched one) goes into the black fuse box, but it doesn't, it loops round and goes into the brown fuse box. Really sorry about that.
So to fix this, you can remove either of the vertical fuses in the brown fuse box (as they're normal sized too), and plug your add-a-circuit into there. Then plug the fuse you removed into the spare slot on the add-a-circuit. Alternatively you can get a mini add-a-circuit and use a spare mini-fuse slot.