Issues with who owns or maintains a boundary, or if one has been moved, are civil matters which need to be resolved with your neighbours. We do not have any jurisdiction in these matters.
Your neighbour may not know how the problem is affecting you, so try and discuss it calmly with them, explaining what the issue is, how it upsets you and how you would like the matter resolved.
You can visit the Land Registry website to obtain maps of boundaries and find out who owns a certain parcel of land. The Land Registry can be contacted using the details below.
If the problem persists you can seek professional legal advice.