Also research 'statutory status' a term meaning that when the land was acquired from the original landholder it was for the express purpose of allotments.
The statutory status can only be overturned by the secretary of state and the council have to find allotment tenants another site instead.
We had a similar issue on our site when someone on a planning committee suggested using the allotment site for housing. The parish council is a combined council of three parishes and naturally committee members wanted housing development in another parish to their own.
Once the council discovered how complicated the laws governing allotment sites were they dismissed the planning committee's proposal and have now found an alternative site for housing so the allotment site is safe - for now.
Beersmith is right. Don't make it easy for the council. If you don't agree don't sign.