Unfortunately I spoke too soon. When I tested it again a bit more
 extensively, it suddenly stopped working. I am going to have to solder in
 some wires to pins 2 and 3 and see if that helps.
  
Removing and then resoldering J3 (the smaller of the two ribbon cable connectors) seems to
have fixed it this time. It is managing to continue to operate for rather longer than
before. Fingers crossed I have resolved it this time. I will test it for longer tomorrow.
Regards
Rob