I'm happy to say that this problem has been solved and the culprit was a slowly dying car battery.
Towards the end of it's life the car would completely cut out whilst turning over, the clocks would reset and I would get all the warning lights on the dash appear. I took a standing reading at 11.4V so it was definitely time for a change.
I managed to source a top of the range Varta Silver battery for it, put it in a couple of weeks ago and since then I have had no more yellow lights
I'm guessing the electric steering on these cars draw a lot of current from the battery and if the battery is weak or dying it could cause strange fault codes to appear.
Morale of the story is check your car battery before forking out on an expensive steering rack replacment