96 astra estate frequently locks itself when door is closed?

Hi ,I have a 1996 Astra deisel estate 1700cc, which frequently locks all doors simultaneously,when one is closed.Sometimes the drivers door and on occasions the tail gate door.I would appreciate your advice on what could be the cause and if possible the solution.Someone said it could be the central lock cpu but I cannot locate it in the car as the manual is unclear as to its exact location.
Thanks,
Chris.