Key less entry problems on a UK 1998 S70
 

I have a 1998 Volvo S70 TDI, recently the drivers door has developed a very strange fault. The car is fited with the standard uk keyless entry system. When un-locking the car with the remote 3 doors will open, but the drivers door remains locked. If you use to key all the locks function normaly and when locking the car all the locks work fine. Also all the window and mirro swiches work without a problem the only thing that is malfunctiong is the unlocking keyless entry side of the drivers door system. Does anyone know what this could be and how much it is likley to cost to fix?