Boundary issue

[IcecreamRocks]

I wondered if anyone has any knowledge with a boundary issue.
We are currently having a lot of work done to the exterior of our house, unfortunately one neighbour is taking issues with this, one of the problems are that they are saying that our boundary line is there's. It clearly states on our deeds it is ours. Now my question is, could either of our deeds be wrong? And how can I find this info out?
They are not the most approachable people so would really appreciate some help before we speak to them. Thanks.

*boundary line is theirs

It is possible that one of the boundaries is incorrect
Can you ask them for sight of their titles

You could download their land registry documents. Are you building to the boundary, does it need a Party Wall agreement? Could it be your boundary but their fence, or vice versa?

The boundary is an imaginary line and doesn't belong to anyone.
The deeds may refer to the original owner that owned the original boundary feature and must maintain it, but unless you know that the boundary feature/fence/wall is original or was paid for by the owner of your house, it could well belong to the neighbour.
You would have to collect evidence to prove it to a legal standard, or if you want to avoid a neighbour dispute you may want to negotiate.
Good luck!

They showed dh a street plan drawing highlighting the whole of their property saying this proves that all boundaries are theirs, we have the same plan for our property so know this isn't the case.
We have all boundaries written in the deeds, we would ideally like to see theirs, but as I say they are being quite hostile. Big backstory.
Hence wanting to see if we could find this info from another source.
Thank you for your reply, much appreciated.

But all street plan drawings of properties highlight the boundaries - it just means they own the land up to that line, and the scale is so small that it is hard to put any sort of measurement to it. Yours will have the same, and if you superimposed the drawings on top of each other the lines would probably overlap.
On my street you maintain the boundary to the left. You don't 'own' a boundary as such, just the responsibility to maintain one side (and possibly back and definitely front).

Johnd2 has it. How rich are you?

Nobody "owns" a boundary. Its a line between 2 properties. You own the land up to the boundary, they own the land up to the boundary on their side. Do you mean a wall or a hedge which you think is the boundary?