Jim Ball, I beg to differ. trespass is a common law and civil offense, it is not arrestable, and thbe police most certainly won't come to arrest anyone no matter how many times you've asked them to leave - they cant, civil offences are not arrestable. You, as the landowner (tenant has same right) may use as much reasonable force as is necessary to evict a trespasser. If they have caused damage you may seek redress in the civil court, but not otherwise.
Clearly if you land in someones field without permission they have the right to evict you, which could make your return to the helo problematic...
Glider pilots sometimes have problems with bolshie farmers damanding high fees to allow access to recover gliders after field landings.
Clearly you do need landowners permission if you are to avoid costly ransom demands to get your machine back.