W163 (post facelift) ML350 Central Locking Issue

[V8 Disco]

Hi, hoping that there may be an expert or two here that can help... my '04 ML has developed an annoying fault - the drivers door no longer centrally locks. It can be locked and unlocked using the key, and pulling the interior handle unlocks the other doors, however it will not lock or unlock from the fob or the switch on the console, nor will it lock when moving off (the other doors all do). This happened suddenly, without any 'misbehaviour' prior.

I expect that the motor/actuator that locks the door has failed but if anyone here has any suggestions other than 'take to Merc Dealer/Merc Independent' I'd be interested to hear.

TIA.