Some developments.
Not a matter of un-pairing and re-pairing as the functionality is greyed out. Still seems to recognise the phone, but won't connect.
And this morning, the development was that the touch-screen was totally blacked out. Turned on, no display on any of the functions. Radio played, so presume it was functioning, but no life in the display.
Booked it in to the Service Dept (for tomorrow). Came out at lunchtime, turned it on and it was working. But its clearly going to be an intermittent fault, so I'd better get it looked at.
Oddly enough, I wondered from the off whether I'd got a dodgy unit. About a week into ownership, I was driving home one night when the Nav suddenly pulled up a map of Woflsburg and showed me driving on something or other Strasse. As I was in Northern Ireland at the time (!) I assumed that the car was just a little homesick - but it did make me think that I'm going to have probs with the unit.