Thanks Martin. I have in fact changed the tyres to Pirelli Powergy, they were the quietest tyre available for what appears to be an unusual tyre size and speed rating (which is stupidly exaggerated, but the garage wouldn't fit a more sensible speed rating as they said it might invalidate the insurance), and there is a noticeable difference - it is generally quieter.
So now I have a spare set of Bridgestones, to go on Ebay. I was thinking of going down to 17" rims with more rubber between the rim and the road and even quieter tyres, but a new set of tyres is one thing, a full set of wheels and tyres is another...
And I agree with you about the sound insulation: I also have a 15 year old A4 convertible, 3.2 Sport Quattro etc etc and the ride is quieter than the new A3. It has thick carpets and sound insulation everywhere.