Central Locking Problem

My 1992 Transport GT - 3.8 central locking system will, once or twice a year, stop working. You can hear a click from somewhere when either of the door lock switches are operated but the doors will not lock. After a few days the central locking will work again for months and then stop again.

My Climer manual is not much good on the central locking system.

Any tips or advice will be appreciated.::)