Citroen ZX Door Locks?

I've got a Citroen ZX K Reg Hatchback.
Over the past few days, the lock on my door has started to become jammed, and I fear it's going to stop opening any moment. They key is "sticky" and
clicks whilst turning in the lock, and the door handle requires much more force to open the door than before.
Does anyone know how much a new lock would cost (UK)? Do garages charge much to fit them? Am I correct in assuming central locking will still work?
I wouldn't mind having 2 keys - i.e one for the ignition & boot & passenger, and another for the drivers side.
Thanks, Dan.
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