[tonupkid]

To add my halfpenny.
No vehicle cover should be completely waterproof. After a cool night warm air will condense moisture onto your bodywork (think dew). Most mornings you see this on uncovered cars. Covered cars can get just as wet, so you need a breathable cover.
The best covers are both waterproof and breathable (like Gautex).
Oli , I'd be interested to hear how you cover fares after its seen a few months of summer.