Q-Park & Fly is my recommendation. It is on the A4 just after the hotel (Travel Inn??).

Its only a few minutes shuttle to the terminals, shuttle run every 10 mins and if whilst you are waiting for the shuttle you get a fantastic view of aircraft landing on 27R.

I think I usually pay about £33 for 4 days.

They (as well as others) can be booked at:

http://www.aph.com/information/airport_parking.htm