There are two 10mm bolts in the very bottom of the underfloor tray, which hold in the rearmost wall around the battery. This wall includes the clamp that holds the battery down. There's a useful photo
here. A socket on a long extension will be required, and you'll be glad if you put some blu-tack in the top of it. If you have the later once-piece L-shaped tool holder as in the photo, there is a cut-out where you can just get to the bolts. Otherwise they are hidden and you have to get the adjacent foam holder out, which is a bit of a fiddle, but it's do-able with some pulling and twisting. Beware, when putting the bolts back in they are very easy to cross-thread! By the same token, the brass inserts they screw into may come out with them if a previous owner cross-threaded them, and will need gluing back in afterwards.