Thanks for your question.
I understand that you would like to 'acquire' the land you have been maintaining for the last 15 years.
Potentially a person can acquire land unilaterally. This is adverse possession. However there are criteria which must be satisfied.
To claim title by adverse possession, you would need to prove the following:
Uninterrupted factual possession of the land for 10 years (if registered land, 12 years unregistered).
Intention to possess the land during that period of possession.
The land must be in your possession.There must be a sufficient degree of exclusive physical control over the land. What is sufficient will depend on the circumstances and, in particular, the nature of the land and the manner in which land of that nature is commonly used. Broadly, the person in possession must have been dealing with the land as an occupying owner might have been expected to deal with it, and no one else must have done so.
Whilst you have maintained the land is that sufficient to satisfy the criteria. The developer has not acted in a way to indicate that it has abandoned the land. The recent negotiations with the council would indicate it has not been abandoned. In addition, it would appear from your question that you have been in contact with the council regarding the land which would indicate that you acknowledge the developern is the owner.
Therefore I would say, based on the facts in your question, that you would not be able to claim the land via adverse possession. If you now start to act in accordance with the criteria above then you may be able to make a successful adverse possession claim in the future.
I hope my reply has been helpful to clarify the position. Please can I ask you to accept / rate my reply.