Could be the actual door lock itself.

Mine had a faulty micro switch in it and it did all sorts of mad stuff with the central locking.

Is the control modual you mean not the GEM module Dojj is talking about? If it is the GEM one it wont be cheap as it needs programmed to the car, it controls loads of stuff from central locking, how long heating front screen stays on, the way rear wiper comes on when in reverse if front wipers are on etc