You're drip-feeding information. This suggests this isn't a boundary dispute, but a right of way dispute. Many houses have a right of way to a path across the back of a row originally used for coal deliveries etc. These rights of way may or may not be documented on titles and deeds, but if they are then you have a lot of leverage as you can demonstrate a legal interest in the right of access. If they are not, it doesn't mean the right of way doesn't exist, but it will be a slightly harder process, because you will have to show you have a prescriptive right of way.
This is nothing to do with a boundary dispute - it is a potential trespass on your right of access. If you can show that your legal interest in the right of access has been substantially interfered with then you can take legal action to re-establish your rights.
I suggest you go on the gardenlaw "rights of way" forum and read posts in particular written by "Pilman" who is a specialist in this field. Once you have your head around the situation, I suggest you post on there with any plans, wording from both your and neighbours title documents (which you can get from the land registry for £3).
You cannot interfere with an established right of access by claiming adverse possession - a right of way persists for ever unless legally rescinded (even if it is never used)
paraphrasing one of Pilman's posts...
It is important to understand that there are two legal property rights in England and Wales,
These were established by section 1 Law of Property Act 1925.
1) Legal estates and equitable interests.
(1) The only estates in land which are capable of subsisting or of being conveyed or created at law are—
(a) An estate in fee simple absolute in possession;
(b) A term of years absolute.
2) The only interests or charges in or over land which are capable of subsisting or of being conveyed or created at law are—
(a) An easement, right, or privilege in or over land for an interest equivalent to an estate in fee simple absolute in possession or a term of years
absolute;
Your Legal Estate in land, would be the area of physical land edged in red on your title plan provided by Land Registry.
That can be either (a) A Freehold Title or (b) A Leasehold Title.
Your Legal Interest over land, would be the easement that is identified as a right of way in the title wording and on the plan attached to the deed
referred to in your Register of Title. (it might also be in your neighbours title - sometimes it gets missed of one title but persists on the other)
The amount of case law regarding substantial obstruction of a right of way is extensive, but if proven, that would be a trespass against your Legal Interest over the Land, which identified in your Register of Title.
This bit of legalese means that the right of way is a "thing" that you "own", just as it would be the freehold of some land (estate in fee simple in possession) or a "thing" that you lease (term of years absolute). You can "own" (have a legal interest in) a right of way, even if you don't "own" the land across which the right of way passes. The right of way is yours, and interference with it is a trespass.