Well, only took four months, but finally had a chance to play. Thanks for all the suggestions but still not luck.I've managed to confirm that the resistor packs are still there. I actually found I had a faulty SCSI cable, so had to replace that, however I think that came about while playing, as the original problem is still there. I'm starting to think maybe it's a fault with the CDROM drive. Unfortunately I don't have another computer with SCSI to try it on so I'm a bit stuck.
I recently acquired an XSurf card so I think I might try using an IDE drive connected to that instead.
Cheers