From the t&c's:
19.9 What happens if I arrive early or leave late?
There is no extra charge if you arrive earlier than your stated time, or leave later, as long as (in both cases) it is on the same day.
This means you can save money if the collection time is later in the day of your return flight than the drop off time on the day of your outbound flight, because you only need to book for the full days you use. In practice you might as well book from 2300 hrs on day 1 till 0100 hrs on the last day to make the most of it.
The headline on the booking page does say arrive up to an hour early but it's the full t&c's that matter and that says you can have a full day effectively. Don't get too excited as the minimum you can book is a full 24 hour period but it does mean two days for the price of one, three for two etc.
I'm going to test it this weekend. I'm returning 23 hours after my departure so only expect to be in the car park for 25 hours, so I can take the risk of getting charged for an extra hour at the start and end.
It's £25.99 per day for mid term if you book at the last minute. Saving that amount on parking might even get you a sandwich in the departures hall!
Does anyone know if this works at this car park or if there are similar rules at other car parks? I haven't checked with APCOA or the airport themselves but I guess we better keep this quiet in case it does.