I've been using one of these for the last two years:
http://www.cover-zone.com/stormforce-car-covers.htm
Not cheap, but it does exactly as it says on the tin, including the ability to cover the car when wet, so I can recommend one. It is a tight fit, though, as you can see. What really helps are the straps from each hem under the car - won't lift off in high winds.
A cheap tarp from Homebase will only trap condensation and the car will quietly rot away without your knowing! I destroyed an old Brit bike by wrapping it in a plastic sheet over winter, many years ago - never again.