The problem returned after eight days, but this time accompanied by the sort of "click" that occurs during start-up, followed by the sound of one of the two SATA drives spinning up.
A little more digging and I was fairly sure that I either had a faulty MB or an intermittent power supply. I'd not had a SATA power cable problem before, but there's always a first time. I've now replaced the PS (500W) and haven't had a problem so far.
Ray.