thank you. Based upon what you say, the first step would be to investigate with the land registry to ascertain if the land in question is registered. You can carry out an initial search using their online map searrch facility but it may be necessary to obtain a definitive search by carrying out a search of the index map:
in the above, you will establish one of two things. Either the land will be registered or unregistered. If it is unregistered, the rules are relatively simple in that you will need to show 12 years of continuous occupation whereby you have treated the land as your own and excluded others, usually by reference to a boundary fence or similar structure. If you can satisfy this criteria, you can make an application for adverse possession.
If by contrast, the land is registered at the land registry to somebody else other than you, then the rules are similar but a little different. Instead of 12 years, you only need to show 10 years of occupation as above but in addition to the above criteria, you also need to show one further criteria; this final criteria is either that the land was gifted to you or otherwise promised to you in some way on condition that you maintained it or something similar and you can show that the reason you maintained it was in reliance upon that gift or statement OR that you have occupied the land in the incorrect but reasonable belief that it was part of your garden.
Satisfying this final criteria can be difficult and this was intentionally made so in order to reduce the number of claims for adverse possession but were made in respect of registered land based on possession alone was felt to be unfair to the real owners of the land in question. If however you believe you can satisfy this final criteria where the land is registered, and similarly, you can make a claim for adverse possession of the land