Just an update.
I still cannot remove the door card to access the central locking unit. This is proving to be very difficult!
Although I cannot lock the doors using the remote control unit, thy WILL LOCK manually using the key. The good 'old fashioned' way! I have now wired into the system an immobiliser/alarm I had on the shelf in the garage, and guess what, IT WORKS!
So why doesn't the proper Ford remote control unit work? Have replaced battery just as a matter of course - but still no joy.
It obviously cannot be a problem with the O/S door locking mechanism as the other unit I have installed would also not work - would it????