The property deeds or drawings should indicate ownership of the fence with a small ‘T’ mark. If you see an ‘H’, this signifies a shared boundary.
As others have mentioned, even if your neighbour owns the boundary, they are not necessarily required to install a proper fence. Additionally, the position of the rails isn’t always a reliable indicator—while traditionally they were placed inside the owner's garden, in the past decade, many homeowners have preferred having the more attractive side of the fence facing their own property, meaning the rails are on the neighbour’s side.
Boundary issues can become complicated, particularly if there have been disputes in the past. For example, if previous neighbours didn’t get along, the owner of the boundary may have refused to install a fence. As a result, the former owners of your property may have erected one on their own land, slightly inside the boundary.
If I were in your position, I would take a friendly approach:
- Ask your neighbours directly who is responsible for the fence.
- Check with others on your street to see which fences they own.
With this information, you should have a clearer idea of how to proceed.
Have a read of this page, it covers most of it
https://www.jacksons-fencing.co.uk/fencing-advice/who-owns-the-fence