Whether in town or on the farm, it pays to know your obligations around boundary fences.
The Fencing Act of 1978 says if there's no adequate boundary fence between you and your neighbour, both parties split the costs of building (or repairing) a fence. Roadside fences aren't covered, but local Stock Control Bylaws are relevant there, as fences should be adequate to prevent livestock from escaping onto the road.
If you're planning to build or mend a boundary fence, it's sensible to get agreement first (preferably in writing) so both you and your neighbour know what you're signing up for.
If you can't agree, you can serve your neighbour with a Fencing Notice.
The notice should say that it's served under the Fencing Act and outline all the relevant details, such as which boundary is to be fenced (a map may help), the materials and type of fence, how much it will cost, who will do the work, when work will start and what to do if they disagree. Like all legal documents, it should be clearly signed and dated, with a copy kept for reference.
The neighbour has 21 days to respond, otherwise their agreement will be assumed. If they object, they should respond with a 'Cross-Notice', explaining their objections and suggesting another option for the fence. If agreement still can't be reached, you'll both have to go to a court or disputes tribunal to get it sorted.