Specifically relating to farmland, if your occupation started after 1 September 1995 (and you are not a succession tenant) and you have paid a rent you will be on a Farm Business Tenancy (FBT). This does not provide any security of tenure (beyond any fixed term), rents are normally set at a Market Level and you can negotiate the terms of the agreement. This may, or may not, be by way of a written document.
If you occupy land for part of the year simply, say, for grazing, a hay or silage crop, or to grow a single crop of vegetables then you may be on a short FBT, but more likely you are on a licence. A Licence gives no security of tenure and does not give you exclusive possession of the land. Therefore if the Landowner decides to enter the land they are entitled to do so, even if this impacts on you.
For occupations that started before September 1995 the law is more complicated:
If your occupation of the agricultural holding started before 1 September 1995 then your tenure will be covered by the Agricultural Holdings Act 1986 (AHA), an Act that pulled together existing legislation and provided security of tenure for the