Thank you Kelly, Chris, Paddy and Bart but, I'm afraid I've still got the problem.
If this problem hadn't started immediately after I messed up the firmware, I'd suspect that the CD had just packed up and needed replacing.
Kelly - I checked the extensions and they're all there.
Chris - How do I "boot into Open Firmware"?
Paddy - I've reset the PRAM but haven't managed to find my way in, so I've not hit the CUDA thingy yet. The iMac starts OK from the hard drive and works OK in every respect EXCEPT the CD.
Bart - I downloaded and installed the CD Firmware update without success.
Thank you all so far.
If you (or anyone) have any further suggestions I'd be grateful.
Helen