Hi Trevor,
Welcome to the Forum. I trust that for the benefit of others, you will eventually post as to the outcome of your problem.
Please note the following.
1. On my 2006 x244 2.8JTD, the central locking operates on all doors, but can only be initiated from the cab doors, a remote fob, and a recently added switch on the dash.
2, While investigating a lock initiation fault on the LHS cab door last month, I disconnected the battery. I was still able to lock all of the doors individually via the key, or by pushing the individual door buttons down.
From 1 above, if you are trying to lock the vehicle by turning the key in either the sliding door, or the rear door, it will not work.
If on the other hand you are trying to lock the vehicle via the cab doors, all doors should lock. A possible cause of failure could be a faulty fuse, F38, RHS cab fuse panel, bottom row 3rd from left. (20A yellow). Fuses usually blow for a reason, so if this is the case consider the possibility of a faulty lock motor.
