If you were willing to book with a company who are slightly off sight but still offer 24 hour transport between terminal and car, then try "Holiday parking-Supersaver", I found them on the BAA website for gatwick and got 8 nights parking in August for the bargain price of £27.40. They are a certified, approved company so i feel safe but i haven't used them yet. Will write a review on my return. Anyway just a thought if you were trying to snag a bargain,
Claire xx