its a three generation one , not sure what u mean re "protects your rights for a goodly period of time"
Well, if it was a five year one and you’re four-and-a-half years in, they have only to wait. If it’s only just started, you can have your five years, or negotiate an early termination bonus.
However it’s a three generation one, so now the key things are which generation are you, how near to retirement are you, and do you have a successor who’ll be wanting to farm the land. If you’re the first or second generation and or have decades still to go yourself and or have a youngster coming through, then your position is very strong. You can completely block their “sale with vacant possession”, or can ask for up to 40-60% of the open market unencumbered value to vacate. If this seems like a route you might take, you might want to ask a local agricultural land agent for an opinion, and possibly even to act for you in negotiations.
They can, of course, sell the land with your tenancy in place. But if the potential buyer is wanting to use the land, it probably needs to be free of your occupancy. There might be a middle ground; if for instance you only want another five or six years, there’s no one coming on behind, the potential purchaser might be willing to let you have your six years and give you an early termination bonus in order to secure the land for themselves in six years time.