Had another thought - the PSU has two, independently wired, four pin plugs for the CPU. Seems there are some 8 pin power units out there and you have to use both plugs to accommodate them.
I switched the leads in the hope of finding non-stop volts (if they ever did stop - not tried the multi meter yet).
Turned it on, successfully set the correct time and date before it all stopped at 3 minutes 17 seconds.
I creep towards ordering another board, don't I?
Sigh.