The parking system is now Pay for parking when you leave Durlston, rather than paying on arrival. A camera will read your number plate on entrance, you can then go for your walk, visit the Castle, enjoy a meal at the cafe etc and on completion of your visit, go to either of the two Parking Machines, enter your registration number and the machine will calculate your parking charge - then you pay either by cash or contactless payment.
So Pay on Leaving rather than arrival. If you do forget you can pay online then pay online, via Park with Ease website (within 48 hours) for more information