Our council blows hot and cold when it comes to letting out vacant plots, and for years we had a long waiting list but plots which were either vacant or poorly worked.
Recently they have taken the tack that if there is no waiting list (which is now the case for our site) then if an existing tenant want a second plot, especially if is it adjoining their current one, then that is fine; better than leaving the ground to grow weeds and become a liability.
It would seem logical that in areas with high demand, vacant plots should go to people of the waiting list when their name reaches the top, and if the council introduces a new policy that it is only one plot (or half plot) per household that plot holders who have more than one, and work them well, should keep them, and the new policy apply to tenants from the date it is implemented.
As Snowy said, a meeting with the council seems the way forward, but make sure that you have a clear idea of how ideally you would want to move forward in affair and reasonable manner. Please let us know how you get on