Bit of an update.
This year I've put winter tyres on both cars.
The Passat is still running the Dunlop Wintersport 3D's already discussed, on the standard 16" Passat Sport alloys (205/55/16).
The Zafira that we bought recently is running a new set of Nokian WRG2's, that I bought via mytyres in December. They're on a spare set of steel rims that I bought off Ebay. Size is also 205/55/16 (the standard alloys on this model run 225/45/17, but 205/55/16 is an "approved" size for this car and fitted on lower trim spec models).
Both have been excellent, the Zafira is doing very well - brand new winter tyres with full tread depth, and the weight of the car (over 1600kg, big diesel lump at front over driving wheels) are a very good combination. Given that my wife is heavily pregnant and is also ferrying our 17month old son around, it's quite reassuring - over the last few weeks, there have been a large number of times where she's kept going around most other cars that have got stuck up the long hill to our village.
My advice if anyone is going to fit winters - is to still drive as carefully as you would with summer tyres. The difference is you're far more likely to get going (and be able to steer and stop) where summer tyres will just spin and slither. Don't get carried away with over confidence, as they're not invincible.
A word of warning though - when you get pure ice like we've got at the moment, no tyre will going to give you any grip! (Aside from studded tyres which aren't suitable in the UK, that is).