Thanks for swift reply.
(I have the old version of the board) I checked out all the diodes and they appear to be ok and double checked the 470R and 4k7 resistors around IC5B - those are correct too (I was hoping that I got them mixed up ;-) )
You've certainly given me an idea of what could be wrong and that's a great help. Any other suggestions would also be appreciated.
Thanks for your time on this,
Regards,
Trevor