probably failed door lock assembly, about £75 from a main dealer, takes about 1/2 hr to change. might be worth getting the door inner trim off, getting the lock out and giving it a dose of wd40 or whatever your preferred lubricant is (not KY). also lube all the rods while you're at it. maybe it'll cure it, maybe it won't, but at least you'll know how it all comes apart and goes back together when you buy the new one.
good luck.