Just to follow on, the unit has been sitting in the cupboard for the last couple of months.
All the above have been tried, but it's still not playing ball. The bluetooth has been removed and the RCD310 put back in to try and it works 100%, but the RNS is still not behaving and the screen/unit flickers on/off every so often.
Would it be worth updating the firmware as a final shot, or if there is something dodgy with the unit is there a greater chance of bricking it?
He intends to sell it as faulty at a loss as the owner won't give him the money back, but I don't know whether its worth taking a gamble with the firmware or selling as is.