Escort central locking fault

Model 1.8 Diesel 1995.
My problem is that sometimes after parking & switching off ignition I find that all the doors are locked and I can't get out.
I can clear the fault by restarting the engine, or locking & unlocking the drivers door from the outside. some times this ritual has to be repeated several times.
Can any one give me any clues as to what might be the cause ?
Thank you Mike S.
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