To quote the law according to the Allotments Act: "It is the duty of EVERY allotments authority, where they are of the opinion that there is a demand for allotments in their area, to provide a sufficient number of them, and then to let them to persons resident in the area. If the duty is not carried out, an interested person (a prospective tenant for instance) would be able to apply for a judicial review and seek a mandatory order in accordance with Part 55 of the CPR."
Further to this, the Act states that"The authority MUST take into consideration any representation in writing made to them by any six registered parliamentary electors or council tax payers resident in the area".
So, your council is obliged, by law, to provide sufficient allotment plots to satisfy demand.
The valid reasons a council may give existing tenants notice to quit are many and varied, but they are absolutely NOT allowed to terminate an existing tenant in order to install another one. The only exception to this is if said existing tenant has persistently broken site rules or who has not paid their due rents.
A book titled "The Law of Allotments" by Paul Clayden has proved very useful on the past on our site, and I would suggest it would be worth your investing in a copy if it is still in print, or perhaps another such book which lays out the Allotment Act in detail.
Your council cannot simply do as it pleases.
