Well guys
I have had no occurence of cutting out since fitting the new PSU.
Also overcome the BIOS flashing issue.
I disconnected all drives except the IDE boot drive and floppy and flashed, and it updated.
I had to re-set some of the CMOS settings but it took it and booted up Ok.
The Dual BIOS on the mainboard is a great feature. The ability to fall back to the original if things don't go quite to plan alleviates some of the fear I had with "tinkering" with it.
Anyway once I was confident the updated BIOS was going to stick, I re-connected all drives set the CMOS, and wizzo the pc was singin' and dancin' again.
After a few days and nothing adverse having happened I gathered courage and flashed the backup BIOS to the new update.
Cheers guys
Billy