Why I asked about declaring a change in wheel or tyre size to a 'winter spec' is because on the A3 forum it is noted that some insurance companies are stating that even factory fitted options are modifications that need to be declared. I was speculating that a change in size of wheel and tyre is a deviation to the spec the car was supplied from the factory thus this needs to be declared also. Fair enough if no one agrees but insurance companies do have a reputation of worming their way out of paying for a claim if they can.
Fair enough point. I think that in the long run you'd be able to argue if you're using factory specification tyre sizes with more appropriate tyres for the conditions that they should pay out. Plus, as I had said before, provided the sizes were correct I can't see how they'd have a leg to stand on as tyres are consumable parts and they can't refuse if you replace with a different brand for example.