Sounds like the central locking actuator in the driver's side door. Obviously, check the basics like fuses first but because the doors all lock when using the switch on the inside of the door, it sounds like the actuator. you can but them on e-bay and from parts suppliers for around £20. To replace the part, you will need to remove the door panel from the inside of the door to access the actuator. there should be guides available online and also youtube videos to help.