I'm fairly sure you can't charge them money without displaying some kind of scheme of charges but that would also require a change of use of the land and that makes it more complicated.
You don't necessarily need a change of use if the land is used for a different purpose for less than 28 days a year.
In some circumstances you must have a price list on display (i.e. in a pub or a bakers) where this is required by statute, but if a trespasser with an aeroplane wants to take off after arriving without permission you are free to offer him permission for a fee that you make-up on the spot and impose any other conditions - such as at his own risk - as you think fit. The trespasser is equally free to reject yout offer and not take off. It would be wise to give a recipt with the conditions written on it - and keep a copy.