One cable attaches to and operates the outside door handle and the other to the inside door handle. The lengths and ends of the cable differ. See links below.
Are you having trouble opening the door from the interior and exterior handles?
If exterior, do you have the time and ability to remove the interior door card and remove the exterior door handle? As the issue could be due to slack in the cable or due to deteriorated cogs in the handle mechanism. Removing the handle and carrier will help to pin point the problem.
If the problem is with the interior door handle then it would most likely be a case of changing the cable - which is far less time consuming than dealing with the exterior handle!
thanks for taking the time to explain all that mate. I've had the door card off before and it definitely isn't the interior one.
The problem seems to be more related to the central locking system after doing some more research. Very odd indeed. But after what you posted and after reading some more, I may need to investigate this some more.