Hi RipperRoo, good to hear from you
A stuck key (or even an utterly dead keyboard) wouldn't stop a post would it? Wouldn't I get something like "keyboard error, press F1 to continue"

[funniest error message ever, methinks]
I only get the post with the cmos jumper in the "clear cmos" position, although I haven't tried removing it altogether (will do that tomorrow) and in that position I can't get into the bios, no, there is no responce to the keyboard at all. (I can try another keyboard tomorrow though - tonight I'm dedicated to this beautiful bottle of merlot)
You are thinking on the right lines though, I suspect, and have restarted my thinking processes (was brooding in despair for two weeks & have hidden the offending box out-of-sight) so thanks for that