Hi Paul,
My 996 is always kept on the drive - so I bought this: http://www.classicadditions.com/index.php?cPath=47_48&osCsid=54f4979954cd01363faed51c25464ad3
It only cost me £110 - with a shoulder carry bag (tell them you're a PCGB member) and has lasted me 2 years. I wash it at the laundrette every 6 months or so and the water proof and breathable qualities are like new again. I was going to get the heavier version (which isn't breathable) but this is better as you can even put it on the car when it's wet (if you really have to) and the car will dry out underneath it. As long as your car is well waxed (which most are) this is a great cover.
I'd be very careful of the OPC ones; I don't think that they're designed for outdoor use - plus they tend to be a bit of an advert, If you know what I mean!?