Thanks for the contributions and advice
The only reason I considered fitting a set of 17" was because I need to fit a set of new tyres shortly and I was offered a set of Audi 17" alloys with tyres at the right money - I am aware of the importance of maintaining correct tyre choice and I noted other A4s run on 17s
I am not fond of fitting spigots to the wheels and I will probably stay on the 16s and get a set of tyres - with regards to insurance, I guess you could call a wheel upgrade a "modification" and I am dammed if I am paying them any more money on my policy - Insurance companies are like banks and I don't think many of us like the bank !
Paul