It's more a contract than a license, and usually runs from some archaic time like Lammas to Michaelmas. Your solicitor could arrange it for you, or your local agricultural mart, which often also deal in land rentals. It's certainly a good idea to have a contract, as the grazer has rights and you also have rights to get them off when you want the field yourself. Make sure you mention fence maintenance, pasture care and include a clause that says the animals grazing must have been appropriately wormed, etc, or your land could end up dirty by the time you want to keep your own livestock. Avoid horses as they wreck the ground!