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I am afraid you are not entitled to physically remove the fence as you could be guilty of criminal damage, however daft it sounds.
The best practical way to resolve the problem is as follows-
Once a Buyer has been found, your neighbour's Solicitor will send her a Property Information Form to complete. This Form answers various questions concerning the property, including a question as to whether or not there are any disputes over the property. If a Seller lies on this Form, their Buyer can sue them for misrepresentation.
Likewise, Estate Agents are under a duty to notify prospective Buyers of any issues they are aware of that may put off a Buyer from purchasing the property.
Therefore, if you have no joy from your neighbour or son in law, you should write another letter to your neighbour and also hand deliver a copy to her Estate Agents, so it puts them on Notice of the dispute.
The Agents will then have to disclose the issue to any prospective Buyer and will hopefully mean your neighbour will do all she can to resolve the problem as no Buyer will want to buy her bungalow while there is an ongoing dispute.
I hope this assists you and sets out the legal position.