lol
what's the old saying? "If it ain't broke don't fix it." And I don't think "old and original" = "broke"
Hah.
Well, I had spent some time last night fault finding, got as far as only one RAM module in, no cards of any kind, and I tried a CMOS reset; no joy.
Tonight I took it further, no RAM at all, disconnected both drives, tried a longer CMOS reset cycle. Tried loosening and reseating the CPU heatsink. Still no joy. Then I looked at the BIOS chip. In its socket. And I pressed it with my finger; there was that creaking noise you get when a chip realigns itself in a socket. Ah-hah! And hey presto, when I press the power button this time I get an annoyed sequence of beeps which I am pretty sure means "where are all my RAM and stuff?"
So tis fixed.
Sockets should be banned. Having a real BIOS and an emergency backup BIOS chip seems like a more reliable solution like what my Windows PC does.