Ordinance survey map, 1:2500 scale. Shows exact acerage in each land parcel.
Areas for letting are notoriously vauge and often incorrect. Hedgerows are not normally deducted. Larger areas of bog, rock, water, yards, etc may or may not be deducted. i.e. a 20 acre field which includes a 1 acre yard and 3 acres of bog may rent as 20 acres at a somewhat lower rent per acre. In my opinion, if you are renting out 20 acres then state it is 20 acres. Agree the yearly price and divide this by 20 to get your per acre price.