Thanks for the replies, WD 40 proved to be a temporary solution but then unfortunately the rear door lock changed it's mind and instead of being open all the time it became locked all the time. The result was that I took the car to a local independant body shop who managed to remove the door trim and open the door (I didnt ask how ) and replaced the complete door lock,cables,inner handle and latch plate with genuine parts at a sensible price.All works perfectly now.