Thanks to Reverend, Dehcbad25 & rik
Unfortunately, none of those things were viable, and we've had to resort to the "good" 'ol backup and reformat game. I guess this is what people get for not creating their restore disks like they ought to!
rik: there was a hidden partition on the HDD but it was a diagnostic partition only. I booted into it & poked about, but found it was only for hardware diagnosis - no restore options available anywhere.