Hi Mat,
I bought an outside cover in 2002 from Classic Additions for £150. It had a soft felt like material on the underside and the outside was frost, UV and basically everything proof.
I glued two wine corks on the underside of the cover under the breather holes as the rain will pour through due to the dip caused by the wipers sitting near the holes. The cork solved the problem and they never came off!
hooking it on the rear is easy and stops curious people from being able to easily take a peak. The front I hooked on the the front nearside arm that connects the front fibreglass near the wheel arch to the chassis. A 'belt' strap is supplied to run across the central underside of the car. I used it once when a strong winds were forecasted but generally if it is hooked on it will withstand most winds.
It is the size M and is the best fit although it is a little baggy saggy around the rear.
This is where I used to live in High Wycombe and the covered car can be seen on Google Earth:-
Now the bad news. The cover comes with a one year warranty. After around 18 months I noticed that the top part of the cover had become slightly lighter in colour and was starting to crazy pave along the top due to the UV rays and other elements. The rain found its way through the gaps to the soft felt like sponge material which soaked it up like a wick. I used to try and fold the cover up carefully when taking it off the car but the extra folding probably shortened the materials life.
I would see what covers are on the market now that offer the important UV/Frost protection but with a better warranty or least a cheaper outright cost. To summise it was a great cover, a shame it didn't last longer. Luckily as it was beginning to fail I was buying a house with a garage so I no longer need one.