thanks to all for your suggestions :)
I've tried them all, including restarting the Mac and holding down on the mouse, as well as the Terminal command[s]
I've also tried connecting the Superdrive to my iMac and repeating all the possible solutions as well as using the 'eject' key on the Magic Keyboard. When I issue the Terminal command, I can hear the drive trying to eject the disc, but nothing more happens.
It doesn't show up in Finder on either machine.
I suspect the CD is somehow jammed in the drive, so is probably only going to be useful as a coaster if I can ever get it out. Unlike standard CD drives, there doesn't seem to be any 'emergency' manual eject - which takes the form of a hole to stick an unfolded paper clip in.
I can't even see how to dismantle the drive [which is presumably dead meat now] so I can at least retrieve my CD.
Any suggestions welcome!