Definitely a planning issue having worked as a ‘planner’ for the last 13 years. Contact the local authorities planning enforcement officer and complain to them. They should visit the offending party and advise them of the options they have. The local authority may serve them with an enforcement notice, which they can appeal against, they can also apply for planning permission for the fence which can take up to 8 weeks and if refused they can then appeal that decision which can take up to another 6 months. Or they can reduce the height of the fence to 2 metres which is permitted development unless the property lies within a conservation area or is listed. It can be a long drawn out situation but you will have an opportunity to comment on things but only on planning matters. If you need any help more than willing to give you some further advice email in profile and good luck