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1. Dear Alex, can you please clarify a few points. Where your neighbour wants to put a gate, who owns the land on either side of where the gate is going to be placed? Is your neighbour proposing to have access over your land when she has access through your boundary fence? Does your neighbour have any rights of easement, such as a right of way, over your land?
2. I am sorry, Alex, but you did not answer the questions I have asked. What you have said is totally confusing and makes no sense. It is as clear as mud. If you wish I will OPT OUT and some other Expert can answer your question. Alternatively, if you wish, you can answer the questions I have asked and I will attempt to answer your query.
3. Quite simply, if your neighbour owns the land on both sides of the fence, then she is free to do what she wants with it, including putting an access entrance in it, so long as this does not interfere with your exercise of your easement. An easement is only a limited right. It gives no ownership of the property over which the easement obtains. So, you cannot prevent the owner - if your neighbour purchases it - from doing what they want with it, so long as this does not prevent your exercise of your easement. So, your neighbour can put an entranceway in this boundary fence as she now owns the land on both sides of it. In fact, she could take away the fence altogether!