I have limited sympathy with Porsche on this issue, if they had taken a pro-active view on the environmental effect of their cars they wouldn't be in a position where they are having to fight this.
Look at BMW for example; you would have to by a 540i before you go over the CO2 £25 a day limit. Even the X5 3.0d is okay for £8 a day. If BMW can manage to get such vehicles down to these levels why can't Porsche? It just highlights their lack of environmental concern which is highlighted by that gas guzzling Cayenne (which, I might add is rediculously urgly to boot), they don't even offer a mildly frugal version of it!