The central locking solenoid in your door is shagged.

You're looking around the £70-£80 mark for a new one and it's a bit of a pig to do as the metalwork inside the door (once you've managed to get the doorcards and that shitty insulation stuff off) is razor sharp, the access is ridiculously tight (even more so on a 5 door) and you've got loads of little pushrods holding everything together so it's a fiddly little fucker of a job!