A problem with these is the inner escutcheon on the door handle seizing causing central locking problems,you have to remove the door card for access,look at straight behind the outer handle you pull,it acts on a lever with a pin and spring,the pin sometimes corrodes and rusts up causing the seizing/sticking and can sometimes be freed off with lubricant,sometimes it is easier just to replace. Other problems were the actual lock/latch assy in the doors but the seized pin would be worth a look,it may need to go onto a computer to actuate your central locking manually to pinpoint which door is giving problems,good luck
cdc
April 2014