Best thing is get a diagnostic check done to establish which part it is. This will cost around £60. If it's the drivers door locking mechanism then I think you'll be looking around £120. I say this, as I paid that much to get a similar issue with my central locking fixed which was due to the drivers door lock going faulty. This job.was done by an auto electrician.
I'm unsure how much it would cost to replace the central locking module though, sorry.
Cheers
Steve