Thanks guys for recommending Chris. Good service, and very quick turnaround like you say. Although one of the keys had a new circuit board, one of them was absolutely fine, and still disappointingly neither of them work the remote locking. I tried disconnecting the battery for an hour or so, and this unfortunately made no difference.
Obviously the immobiliser system is separate from the central locking and alarm, and the keys are fine in respect of the immobiliser - they both start the ignition. I'm assuming that the wiring between the control unit and door locks is sound, as the keys both trigger the central locking when physically in the door lock.
My educated guess is there is a something wrong between the receiver unit for the radio signal and the central locking and alarm control units. The LED and push button on the receiver (located in the interior light unit), both work and let me code the remotes (from the code card) to the car, so it would seem odd if this itself had died.
From looking in the workshop manual I can't really work out what's wrong.
I am, as they say, up creek without paddle