Thanks Toscarelli and Charles for their advice. Some of the screws I thought were just holding the drive were in fact holding the lid down. Now I can inspect the inside..mmm dusty!
An EPROM, presumably the operating system, has a handwritten label that says 'c 1986 SCI 2000 V3.0'
I'm now goning to dig out that can of compressed air.
The disk drive is by Canon. Anyone got any samples to trade? I have lots of 720K disks to swap if anyone needs blanks (I am an ex-amiga computer owner).
Thanks again, Keith.