Many thanks Fruit Bat. I removed the memory chips (with an anti-static wristband on). Whilst I was inside I did a bit of a service by hoovering up dust. Booted up using original RAM chips, then installed 2nd, booted up fine, shut down added 3rd and booted up again.
BUT the same error happened when I plugged in my printer. I got another spooler error and the printer had sprung into life and was printing out the same document again!
So, my question is, do you reckon this is a symptom of the hard drive going home? Shall I add a 2nd internal drive, just in case? What would you do?