I write as a former Council Officer, with some awareness of how their systems work.
1. If you are denied information, and live in the area of that Council, you can submit a 'Freedom of Information Request' for information about what land the Council has rented or sold to whom. They can only withold that information if it is commercially sensitive to the tenants/purchasers which, if the other owner/tenant is a private person, seems unlikely.
2. Failing that, you could ask the local Newspaper to investigate, on the premise that the Council is selling Public Assets (or renting them) without declaring them.
3. There may also be Minutes of the Council Meeting where it was agreed that the land could be sold, or that a nominated Council Officer had permission to sell that Public Asset. Most Council Meeting Minutes are accessible to the general public, so you can sess them. The central Library usually keeps a copy for you to reference.
4. Your local Councillor could be an asset to your cause; the Council can give you the name and contact details for him/her.
5. And if you find that the information is all fudged and unhelpful, you can ask your Councillor to table a question regarding the sale of Public Assets (in the form of spare land) that might benefit the Council's coffers. Unless the Council has scheduled use of that land in the District Plan, or has a prospective buyer already, they can't deny your application to buy it.
6. But it might take the Councillors to push the Officers to do anything if they are being intransigent.
7. Councillors are inclined to blame their officers for not making decisions, or making bad ones. Council Officers are inclined to blame the Councillors for not having authority to do things they themselves don't actually want to do. The 'oxygen' of publicity and persistent enquiry (amounting to harrassment, if necessary!) can sometimes un-bung the intransigence. You have nothing to lose!
Good Luck!
John