Paul,

Thanks for this reply. Performed re-test using both PSU's having first
checked, off board, both working and all connectors showing correct
voltages. Tested both with data and power connectors to 2 x DVD connected,
then again with data and power connectors disconnected.
Original fitted PSU powers up (well fan and fan lights at least) when ON
button pressed in both test cases. Spare PSU doesn't !!! (again in both
cases)

I can't think of any reason why this is the case as both tested fine off
board.

I've run out of ideas now in trying to resolve original problem other than
coming to conclusion that the moth board is faulty.