Hi Luke1981,
I know this has probably been fixed by now but...
I have had the same issue with my A6 C7. It all started after I drove through a puddle on a rainy day. Initially the error message said something along the lines of "steering system fault: you can continue driving" but the following day after driving a couple of miles down the road the light on the dashboard when from yellow to red and the message changed to something like "Do Not Drive the car". I then took it to the mechanics and they found an electrical issue on a module on the steering rack.
the whole steering rack had to be replaced as the faulty module is not sold separately.