Ford Fiesta central door locking - unlocks

1997 ( R Reg) 1.4litre Ford Fiesta Ghia X 5 door. Vin WF0AXXBAJAVG13930. 85k miles.

Central door locking but the remote key fob has not worked for years despite changing battery and trying to reprogramme. Therefore have been just using the keys in the lock for many years. Now, whenever I lock it, it locks but then unlocks all the doors immediately. Happens if use either driver or passenger door.

Have replaced the door lock (the catch & solenoid (?)), but not the barrel, with one from scrap-yard. This worked perfectly for perhaps 20 attempts with the ignition off before I started the engine and drove it. Then I drove the car and the problem has returned. The lock mechanism is the one with a blue 6 pin electrical connector, not the 4 pin connecter which I have found on lower spec (LX) models.

Have tried to initiate the ‘self-test’ procedure but without success. The plan was to give the car to impoverished son, but for that it does need to be lockable. Clearly it has little residual value, so am looking for a cheap solution. If that meant foregoing the central locking feature, or having two different locks and operating them with the keys, that would be OK (as long as the rear doors could still be locked).

Any advice would be welcome