Any experiences with arriving at a LHR car park the next calendar day against the reservation arrival date?
My assumption is you can arrive any time during the allocated period. I arrived early the other day. You're not supposed to arrive more than 10 minutes early, realised I'd booked 11am, turned up 9.50am and it let me in and I didn't get charged anything extra.
You can arrive 05:00 the next morning and the barrier will recognise you're within your booking.