I keep a spare set of 4 alloys with winter tyres in the garage and have just fitted them myself ready for the summer. I agree that it's best to put 4 on rather than just the two front driven wheels... I have Uniroyal on the back and Westlake on the front.... Last year it really helped in the colder weather and the snow we had up here in North Yorkshire!
Thanks though I seem to have fixed it (fingers crossed):
According to most the only way to cure it is either clean connections or replace switches which involves removal of steering wheel and airbag etc etc. I didn't fancy all that hassle so I looked down the sides of the switches and could see 4 very small square holes. These holes are just big enough to get the end of a small straw down so I sprayed down each one with contact/switch cleaner. This I did a week ago and normal operation was resumed instantly and has worked fine ever since at all speeds... Worth a try for a 5-minute fix!