Chap called Greg runs Coveryourcar.co.uk and I've had several covers from them which I've been impressed with.
I have a Monsoon cover on my Twingo 133 which is a slightly rubberised material and heavy so it doesn't move around. I had the car under it for 3 years or so without removing the cover something I was aware I should have done but the paintwork was absolutely fine.
I had used a Stormforce cover which is a lighter material but I used to find that it would move more or corners would life up. The Monsoon cover is a lot heavier and in turn never moves around.
In terms of using a cover, many detailer types would recommend against them. I've been into detailing for years and I've also used car covers for years as well. All I will say is, prior to covering make sure your car is clean, really clean and dry.
When you fit the cover, make sure you secure the straps underneath tight as it will reduce any movement.
Also in terms of using Coveryourcar I can vouch for their customer service. I bought a cover which I didn't get round to using for 3-4 months, just kept it in it's carry bag until I needed it. Came to fit it and realised it was the wrong one that they'd sent. Considering the length of time I'd had it I wasn't expecting much but they asked for the codes on the cover, arranged a courier for the old cover and sent me the right one.
I uncovered my Twingo 133 in the summer last year and all it needed was a hand polish and wax.