I guess being in North Yorkshire your winters will be harsher than ours, with ambient temperatures lower for a longer period of the year, so winter tyres make sense. The problem other owners running 21" wheels have is sourcing winters of this size an acceptable price, I fact I don't believe you they are available for the wider rear wheels. Winters for my 20" wheels were running at £1,500+
To me it feels wrong to spec a car on 21" wheels to get the sporty look, that your car definetley has, to then fit and drive on 18" wheels for around half of the year.
None of my friends or family down here in the Mediterranean-Midlands have ever purchased or considered getting a separate set of winter wheels - official stats seem to imply less than 3% of UK drivers fit winter tyres, guess most are 'up-north' - MB don't even offer this option when purchasing a new car.
See yours is on 21" wheels, will be interesting to hear what tyres its on and how it drives - every owner on these wheels that I have talked to has the fault, but I'm probably only bumping into the unhappy ones
Great looking car
Tony