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Thanks for your enquiry.
I am pleased to say that there is absolutely nothing stopping you from erecting a fence on your side of the boundary, and your neighbours are not entitled to object to this.
The only thing I would say is, that if you do this, there is a small risk of there being a dispute in the future as to where the true boundary line between the properties lies (your neighbour could, for example, remove the existing fence straddling the boundary, and try to claim the tiny strip of land between the boundary line and where your new fence lies.
I hope this answers your question and sets out the legal position.