Strictly speaking, what you have is the freehold reversion. It's value depends on the state of the site, what obligations you have to your tenants, how much rent you get from them, how easily you can get evict them, and how easily you can do anything else with the site.
At one extreme, if the site is let to an association on a long lease and you have an obligation under the lease to maintain the boundaries and supply them with water, all for a peppercorn rent, and some of the tenants have been using their plots to bury asbestos, and others have set up home in their caravans then you've inherited a huge liability - bad luck! At the other extreme, if the site is in the middle of the town and the tenants are all on a year's notice, and there are lots of other allotments available locally, then there's very little stopping you from kicking them all off, and selling to a developer for several million.
