Update:
Isn't it fun when you get two faults in tandem?
Removed SCSI stuff and changed jumper of HD1 (on IDE2) to master.
Switched on.
BSOD
Left the tray out of caddy.
Box booted normally.
Replaced SCSI.
Wouldn't boot properly, though it did get there eventually. [5œ]
Tried shufflng SCSI HDs.
Either it hung during BIOS detecting things, (1 HD connected - either one) or it booted reluctantly (2 HDs fitted).
Removed SCSI again - booted normally.
Changed HD1 (IDE2) tray for an old Win98 one jumpered as master.
Booted.
Knoppix now boots and runs from CD without demur. It seems the fault was on the SCSI card, not the graphics, as suggested, and the HD on IDE2 was borked.
Not a supply spike or suchlike causing the problem, I'd think, as I run through a decent UPS.
Thanks for everyone's input, and all URLs noted: some (if not all) diagnostic [6] tools will be downloaded and stored on CD.
[5œ] but it took five and a half minutes about it. [6] I don't believe in Di, or Di doesn't believe?