@WayneCampbellsSidekick down load the two filed plans, yours and hers the cost is £6.https://www.gov.uk/search-property-information-land-registry
Establish who is responsible for the boundary/fences and decide if you want a fence or not.
Copy the plans and send them to neighbour with a covering letter.
Dear
Please find enclosed a copy of the Land Registry filed plans showing our boundaries. The plans clearly show the boundary is ……
The previous owners have advised us the fence has been incorrectly placed completely within our boundary. The previous owners confirmed they advised you of this and asked you to correctly reposition it on the boundary. You have failed to do this so we are planning to remove the fence on (date) and reposition it so it sits on the boundary as set out in the deeds.
Or
We are not planning to replace the fence and will return the fence and posts to you. Alternatively we are happy for you to reposition the fence on the boundary by (date).
As previously stated on (dates) we have verbally advised you the piece of ground (add description) is completely within our boundary. With immediate effect we are withdrawing the access you requested to maintain your property. You misrepresented the reason for requesting access and have misused it to tamper with our property. We will be clearing the weed-matting and stones you placed on our property without consent and will return them to you.
In future, if you wish to request access to our property to enable you to carry out repairs and maintenance we would appreciate the request in writing clearly setting out the access required. We will not unreasonably withhold permission but we do not want access to be misused to tamper with our property.
Please let us know by (date) if you plan to reposition the fence.
Happy to discuss
Obviously tailor the letter but basically this is a firm letter establishing the extent of your property. Returning the fence, matting etc. means they have their property back.
This letter on its own would not be classed as a dispute. How they chose to proceed may turn it into a dispute.