We've just returned and did this at the start of our trip. We parked late Fri afternoon and I said when I dropped the car off that I'd be a day later than the 15 nights included as our flight came back early hours (7am) Sunday (today yawn!) the attendant put it in and no extra charge was coming up so said all good and not to worry! May have just got lucky but otherwise I had been expecting to pay the additional nights charge which I believe is £15. Hope that helps!