Um, how can I say this?
I'd had the front of the case off to replace the CD drive (and the replacement is t*ts up!!). On replacing the front, the power switch did not mate properly with the button on the case front, so with the front fully home the power switch was permanently depressed. Having disconnected everything, removed the mobo from the case and attached it to a PSU on the bench, all worked perfectly. Gradually rebuilt it in the case, and re-attaching the power switch lead to the mobo revealed the snag.
Just got to replace that CD again now. Ho hum.
Thanks chaps!