Question Author
Albags - Now you mention it, there was a problem a short while ago with rainwater finding its way into the drivers footwell which I traced to a rubber seal which had perished, situated at the base of the A pillar. For the water to get to the footwell it would have run down the inside of the door jam at the point where the cable bundle runs into the drivers door. I've replaced the perished seal and the footwell is now bone dry, but there could be moisture inside the cable connector - I'll check that out, thanks.
Theshedman - I've looked into your possibility but it's not that I'm afraid (start with the simple solution first). Also just to mention, the fuel cap and boot are not controlled by the central locking, so I can rule those out as well. Thanks anyhow.
BOl10X - I'll consider your idea when checking out what Albags suggested. I've a feeling now the problem lies in the area where the cable bundle goes into the door through a flexible connector. Thanks.