The only thing I can say about car covers is put money into a good breathable one and ensure you allow the car to breath when its a nice day..well as nice as it gets in Britain
When I looked into this for the cab, there are quite a few different types you can buy depending on the room you have and money you want to spend..
The few different "types" I can think of off the top of my head are;
Elasticated cover - full and part covers available - cheapest ones are NOT good for your car, the bet, like C said are around 100 quid and usually made up of three layers
Metal structured one - this covers the car but doesn't actually touch the car allowing the air to flow freely - also can be bolted to the ground, obviously for bad weather..
Inflatable car cover - basically you inflate, drive your car in - mega money and usually used by the concurs chaps/chapess'
Hope this helps
Dinx
