Ok, I've just completed my battery swap and put it all back together. Took some photos along the way, so I'll upload them and describe the tricky bits.
Major findings:
My internal PSU has previously been replaced with a wall-wart and joined to the original power cable with cheap snap connectors. I think I'll re-do it sometime, with a socket on the rear casing, so it's pluggable. I did try to remove all the PSU housing, to save weight, but simply didn't have a spanner small enough to remove the nuts.
I doubt that the battery was my only problem, as the synth "bricked" on first boot, as it occasionally had before. (Display says Welcome and the three LFO lights at the top left are all flashing.) It started up properly on second boot. Let's see how thing go, after I reload all my patches and get to work again!
Cheers,
EG