Problem is now solved ..... turns out, it's the CD-ROM drive playing up!
It was 'just a conincidence' about the different ROM versions.
I booted up 3.0 and it was fine .... upon rebooting (still on 3.0) it didn't work anymore
I then swapped it to the A600 and the drive still didn't work!
The Squirrel SCSI adaptor still works, it's just the drive that's knackered
I've now put 3.1 ROMs in the A1200 ..... I did have to reinstall workbench though, as WB 3.0 seemed unstable in comparision to 3.1 (maybe a library issue?)