So it fired up fine although the keyboard was worse than I remembered, pretty much none of the keys worked. However, with the new membrane in place all keys are now working!
Thanks for the advice on changing the drive belt before attempting to read a disk. When I took out the drive on first inspection the belt looked fine, but as soon as I pulled on it gently it instantly snapped. I ended removing it in about 10 pieces with tweezers.
New belt in place and it seems to be reading (most) disks fine. I did have a problem with Megablasters staying on a black screen after the title screen, but perhaps it's just that the disks are 20 years and past their best (or Radical used cheap disks?).
I'm occasionally getting a purple screen upon turning on, or sometimes the starting screen but with some pixel corruption. Switching off and reseating the cart seems to work, I need to try cleaning the connector as hopefully it is just that.