The MB is a PC Chips M925 and is a micro ATX. It is smaller than most boards and has only one 20 pin power socket. It has no provision for the secondary cable to be attached.
The switch and LED connectors for attaching the cables from the case show the correct position of each, but I tried reversing the plugs and putting the power cable (the one from the case switches, not from the PSU) on the reset pins, even though they are not polarity sensitive, but it had no effect.
I asked the tech support and they suggested that I try to clear (sorry, not flash) the bios by setting the jumper, applying power for a few seconds, then shutting down and putting the jumper into the normal position. Again, it had no effect.
I will try the suggestion of removing the PSU and board and attempting to get some response from the PSU, as you point out, it is a mighty complicated beast. No fuses that I could see, and the board did not work in another case either so I suspect it is the board, not the power.
Anyway, I appreciate the replies.