Thank you Mike,
Will pass this to my garage and get back to you. The car's in there now.
Garage had checked connections and confirmed both were correct (radio and steering) so today they temporarily replaced the radio with another make so I can try that for a while. Fingers crossed.
The radio/CD put in by the garage was a JVC. Not DAB but has not triggered an alarm yet.
You suggest that the problem might be the Sony radio/CD and, after a month, I have decided to go ahead and replace it. Is there a particular make that is likely to make a more reliable connection or is it irrelevant? Might DAB be complicating the situation?