As debdon points out XL are based in the south terminal for all their flights and parking express is the onsite parking for the south terminal, its well signposted and easy to negotiate your way in , debdon's tip on making a note of the nearest bus-stop from where you park is excellent as the last time i parked a bus was coming and i forgot the exact location and when i sent the wife to pick up the car she was cursing me as she struggled to find the car from my vague idea of its whereabouts
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Another thought, i saw that you're driving from Manchester, are you going to book an overnight hotel? depends on your flight times? because you can book the Hilton that is linked to the South Terminal, its expensive but they do hotel and parking combination deals that work out fairly cost effective, try a look at Holidayextras.co.uk, or look at superbreak.com for hotel and parking deals.
Have a good holiday!