PUMA Central Locking Problem

My wife's 2000 model year (nearly 14 years old) Puma (in the UK) has developed a problem with the central locking.
Using either the remote or the key in the door, the central locking goes
through the motions - you can hear whirring inside the doors - but it doesn't actually lock. The whirring seems to be accompanied by a clicking noise inside the front passenger door. When the key is held in the lock position, I can feel a force trying to turn it back towards the unlock position.
If I remove the fuses which relate to the locking system, I can then lock the front doors (one at a time) with the key - but I can't find a way to disable the tailgate button - so the car still can't be secured.
Do these symptoms sound familiar to anyone? Any pointers as to what I need to look for?
I don't think it's a matter of having too little battery power to operate the locks - the engine turns over normally when starting, and there are no other symptoms of a flat battery.
[The Puma shares many components with Fiestas of that era - so any experience of similar problems with Fiestas may well be relevant].
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