Thanks for that. I did a bit more digging online and I think you're right.
I've opened up the unit and taken out the PSU/Drive assembly and I do suspect it's the battery that's the issue.
The battery itself doesn't look too bad on first inspection but I think I can see corrosion on the board - the dark stuff around the battery.
Am I right in understanding that is corrosion? Did you see that on yours?
Also the CPU switcher I mentioned is actually a ROM switcher.
I have found the 68030 that I was thinking about and it was for an old Amiga 500 that I sold many years ago. For some reason I kept the 68030 - a mega midget racer. Still have the manual as well

Thanks again.