It is a common fault on these vehicles for a small spring within the door lock actuator to snap causing the symptoms noted. The original spring within the mechanism is very thinly wound and is constructed from a poor quality steel which will inevitably corrode and the spring will snap. Previously this necessitated the complete replacement of the door lock actuator unit at vast expense, the vehicle manufacturer has not altered the spring design / construction over the years so even installing a complete new actuator would only be a temporary solution.