Well, things went from bad to, er, badder.
A friend suggested 'Archive and Reinstall' would work a treat. Fortunately it archived OK, but the reinstall left me with a blinking question mark folder instead of a booting Mac Mini... it's pretty enough in its own right not quite as productive as I was hoping.
In the end I decided to cut my losses - I could still get to Terminal via the Install disk, so I tar'd everything across to an external drive, and with a deep breath, went for the the full erase-and-reinstall.
Which consistently dies part way through. Hmm. I've tried swapping RAM (getting the case off that thing is still a bugger of a job), but no joy. Maybe I fried something, maybe it's bad Feng Shui, maybe it just doesn't like me. Regardless, it's not supplying a lot of joy right now. The error I get, 'BomFatalError - cpio read error: bad file format' has been thoroughly Googled... but not a lot of luck finding answers. The fact my Hardware Test CD keeps saying I have bad RAM doesn't bode well.
I'm trying to erase-and-resintall thing one more time, let's see what happens. I didn't want to have to buy a new iMac or something ... oh who am I trying to kid, of course I want one :) But I wasn't really expecting to do so a year after getting the Mac Mini.
So my old G3 iceBook is resuming (relatively) active service.
Sigh. Whinge. Moan. Etc.