Hmm, right looks like the part I need is 3B2837016A - a new door lock mechanism for the drivers door.

Interestingly when I searched for the part number, this popped up:
/forums/showthread.php?t=55433&highlight=3B2837016A

Seems SilverFox had the exact same problem I'm having, though I can't PM him yet to ask if the replacement lock resolved the issue!

Any chance anyone can quickly PM him and ask him to have a mosey at this thread?

Cheers!
 
Impressed, ordered a replacement locking unit yesterday, it's just arrived on my desk.

Will try fitting this weekend and see what happens :)