This is potentially about adverse possession or a prescriptive easement.
If someone has used a piece of land without consent and without objection and not in secret for more than 10 years if the land is registered at the Land Registry (12 years if unregistered) they are entitled to have the land registered as their own under the doctrine of Adverse Possession.
Any known owner would be contacted by the Land Registry and an application is made by the possessor and they would be allowed to dispute the application.
Any disputed application is passed onto the Land Tribunal who will decide the issue based upon the evidence.
The has to be an intention to possess the land and it has to be occupied to the exclusion of others which means that fencing is off or possessing it and excluding others in some way is almost essential.
Whilst someone claiming adverse possession will very often have occupied the land for far in excess of the requisite period, it’s very often the case that they have no proof of when they first started to occupy and use the land.
If it was not occupied to the exclusion of others but merely used for a period, then the person using it cannot claim adverse possession but may be able to claim an easement if they can prove 20 years use without consent or objection is not in secret. That application for an easement (such as a right-of-way) would be made under the Prescription Act.
It’s also worth noting that if someone has an easement already over a piece of land, and they fence it off and exclude others, and do so in excess of the requisite period, they cannot claim adverse possession because their possession cannot be adverse if the access was originally by consent as a result of an existing easement.
The time periods are not just in respect of the current owner but the periods of the current owner and all the previous owners are added together.
So it depends how long the next door neighbour has used this piece of land and whether he has just used it or used it exclusively. If it’s more than 10 years exclusively, then he can claim the land as his own or if it’s more than 20 years that he has used it when he can claim an easement by prescription.
All of that is on the strict understanding that he was never given consent to do this in the first place and he just did it.
It is often the case that a robustly worded letter from a/your solicitor threatening a court application and an application for costs, may resolve the issue without the need to go as far as court.
On your house insurance you may have legal expenses cover that would pay for the legal cost so it’s worthwhile checking. Some do and some don't.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have