If a piece of land has the right of way over an adjoining piece and the piece of land with the right-of-way is split into 20 for arguments sake, all 20 pieces enjoy the right-of-way.
This is of course subject to not causing nuisance in common law. Nuisance would be decided using the ordinary English dictionary definition of nuisance, if that use is literally, nuisance.
The problem for you of course is that you can’t beat them with a blunt instrument until they stop doing it your remedy as an application to court for an injunction to stop overuse.
It is not the amount of the use which is relevant but whether it does literally cause a nuisance.
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.
The other issue of course is whether they have planning permission for the separate accommodation and that might be something that you may wish to refer to the planning department at the local authority.
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