Hi Alan,
This could be caused by a number of things. However when I had a similar issue with my a6 it was caused by the drivers door locking mechanism. According to the auto electrician which fixed my fault he said that on some Audi models the central locking system.is controlled by the drivers door mechanism and when this starts to fail it can cause faults with all the other doors. It cost me £120 to fix my faulty mechanism.
Infailing that, it could be the central locking module. But to be sure a diagnostic check could help find the fault or an auto electrician as I used.
Cheers
Steve