If you remove the end cover of the pump ( square cap) underneath youll find a small circuit board.
On the board is an electroyltic capacitor that controls the length of operation of the pump when you turn the door key to lock/unlock.
This capacitor dries up and loses its ability to charge/discharge so the circuitry cant power the pump.
Simple op to remove and replace with one from maplins of the same value...cant remember the value of it now.
Also, another source for these pumps: VW golf series 2 has exactly the same unit to lock its doors and are fairly plentiful in breakers.
Oh yes, the golf also uses the same door actuators.