OK, so 6 months have gone by and my MIDI inputs are not working any more.
I ordered some Caig Deoxit and treated all the connections on my cards and I/O board, but no joy.
Over the years the plastic screws that held the I/O board sandwiched to the main card have slowly been failing for me and I'm down to just one now, so I've ordered some spare nylon m2 bolts.
I'm hoping that when these arrive I'll be able to hold the IO plate in position more tightly and MIDI might come back.