The central locking is now fully working!! Yipeee.
From within the car, switching the door lock switch I could hear a relay switching somewhere behind the dash. Four hours later after removing most of the front facia I managed to find it. The door locks are controlled by a small unit which also controls the interior lighting. (the position of the "black box" is directly behind the small recess on the RH side of the dashboard, the place where you put your sun glasses. To remove it you need access from directly above which means stripping out much of the facia)
Having found the controller it was evident that all five door locking motors were controlled by the same two wires leaving the box. This meant the fault was not with the controller itself.
The final fault was found to be a broken wire between the door and the car body (within the loom that flexes when you open the door). Both the front and rear doors suffered the same fault on the same wire (the orange one)
The final fix only took 30mins with a soldering iron but an extra half a day to rebuild the facia. At least I know what's in behind there now...