the first thing i would do is approach a solicitor with the proof that you have maintained it for that amount of time! they will then get the ball rolling.
Hi, My parents moved into a house which had a footpath up the side of the garden but had been incorporated into their garden. My dad always said that if it is not used regularly by anyone for 12 years then it automatically becomes theirs. They have been there 20 years now and nothing has ever come up about it from the council or anyone