From my limited experience in this area, I think there are a few test questions that help to establish your "right". First, was it the landowner's (or then occupier's) intention that this was to be a long term arrangement? I would argue that having been there more than 20 years, it clearly was a long-term idea.
Second, do you - or have you had - unique and sole access to the land? Have you had to share it with any other user?
Third, have you had the responsibility to maintain the land, for example by cutting hedges, mending fences etc?
There is a difference in law between a tenancy and a licence to occupy. If you have had sole use with responsibilities for a significant number of years, and your initial agreement was for an unspecified time, I would place a bet on that being a tenancy and not a licence, regardless of whatever the landlord might think, and that puts you in a stronger position, if you are minded to muscle it out. Even if you chose to relocate, it would be on your terms and not the landlords, and that puts you in the driving seat.