Hi Aidan
I bought an outside cover for my GTD from Specialised Car Covers
Car covers, caravan covers, bike covers by Specialised Covers
It fits really well and allows the car to 'breathe' and dry out if condensation occurs.
The only problem is my mirrors are not in the correct location, so it doesn't fit perfectly. I'm sure you could request where you want the mirrors (depends where they are located on your car?)
It was reasonably priced (about £250 ish) and took about 2 weeks for delivery (they are made to order)
Jas