Oh, I thought 36 or so was the capacity for a mid-cage, so (biggest sprocket minus smallest sprocket) plus (biggest chainring minus smallest chainring). If it's worked on the setup you mentioned then I might give it a try, would rather see if a long cage one shows up first ideally.
Right finally managed to excavate the cupboard to find the mech. It's an RD-M760 (long cage) in really good condition. I used it for a few months at the most before giving up on rapid rise so there's a few minor marks but nothing of significance TBH. would be looking for £25 posted, PM me if you're interested.