Seems to like the sound of his own voice doesn't he..
Never use a contract as it brings 'legal' into the equation and your far more likely to scare a farmer by telling them that they have to sign a legal document.
I often drive around and call in on the off-chance and only ever had one farmer say no. That was because the land in question was a camping site during the summer months.
Approach them with a smile and be polite. Explain why your there and remember your keeping them from something, so don't waffle.
I've taken pictures of finds in the ground and the hole 'after' its been repaired so as to 'show' farmers that we have respect for their property.
Take a few finds with you and make sure you know what they are to explain the historical reasons for you wanting to have access to the land but above all, listen to what they have to say and take an interest by asking questions.
If the person is abrupt or just rude, 'walk away'. No amount of talking is going to alter that persons attitude. Be polite, thank them for their time and leave.