Thanks folks. Changing wheels for winter is maybe a bit more hassle that I can cope with. I may buy a set of 17's and sell the Monza 2's though. I sound so practical it makes me sick 
I have two sets of 18's; one new set with summer tyres, one old set with winter tyres. Twice a year I go to my local tyre place, with a set of wheels & tyres in the boot. In the time it takes me to walk over the road, buy the lads some tins of beer and bring them back, one set has come off & been replaced with the other, and I'm good to go with tyres appropriate to the season.
Can't see how half an hour twice a year is more hassle than could be coped with. But the cost of 8 tins of beer is getting a bit much
You're still only wearing out four tyres at a time, and in the winter, the advantage of winter tyres is massive. Once you try them you'll be sold.
17's just don't fill the arches well enough in my opinion. Don't chop your 18's in, you'll regret it!! Powder coat them, or live with it, the integrity of the wheels is unaffected by White worm. Nag the dealer, they may even change them under warranty.
