Can you operate the lock with the key?
If so and its just the fob that won't work then its probably the central locking actuator.
Once the door card is off - mind the 'orrible sticky sealant - the you can get to it easily enough. Try manually operating the arm and see if it operates the cl.
Again if so you need a new actuator. Ebay is often your friend just make sure the connectors match as there seem to be lots of different sorts.