I'm assuming you didn't see the BBC News report on the subject the other morning, the ABI said that as long as they are the manufacturer's specification winter tyres the insurance have no interest. Basically saying don't believe all the internet rumours you hear
See pages 264 & 265 of the Owner's Manual
Alfa 147 Owner's Manual
Note the specified winter tyre size, and the 'Maximum speed ratings for snow tyres' section on the next page.
For some reason, try as I might, I can't embed images on this forum. But you can click the links
That's all regardless of the fact that if insurers were to be affected by such a thing, your cover would drop to Road Traffic Act cover only and they would still pay out. You just wouldn't get your own
car repaired. For the insurers to withdraw cover completely they would have to prove that the tyre being run above it's speed rating was what contributed to the accident.