Hello and thanks for using Just Answer.
I am Al and am happy to assist you with your enquiry.
I am afraid you have not acquired any rights over the land. As the Freeholder "consented" to you using this land in return for maintaining it, all you have is what is called a Licence over the land, which can be terminated by the Freeholder whenever he wishes.
A party only acquires any rights over somoeone's if they have possessed land by "adverse possession" (ie without the consent or permission of the true owner) for a period of 10 years or more if the land is registered at the Land Registry.
Your hands are therefore somewhat tied.
I am sorry this is not the answer you were looking for, but it sets out the legal position.
Thanks for your reply.
Was this agreement formally put in writing and signed by both parties and witnessed?
I'm afraid that any grant of any rights in land has to be by way of a Deed (ie a document signed by both parties and witnessed). Therefore, the letter you have is worth nothing in legal terms.