take out the door lock mechanism and strip it down, take the cover off it, about 6 small screws, connect it back to the car and fiddle with the little motor inside, you'll probably find it will lock all the dorrs sometimes but bot others. the alarm will also go off, just press the fob, once its doing locks and unlocks consistantly with both the fob and moving the parts then pack it with grases and put it back together(i used vaseline!!) should be fine then for a few years, its not guaranteed but a bit cheaper than a new part.
there are two wires(sets of) that go to the mech, one has several wires and a multi plug and another has just one wire. make sure these are nice and clean and secure.
also check the microswitch on the mech is good and working!
if it works youve save a few quid, if it doesnt youve wasted an hour of your life learning how to take the mech to bits!!!