Just because the sign is metal and screwed on doesn't make it official, they are cheap and easy to get made up. Landowners are not realized a hand painted sign isn't enough to scare these townies off and are getting more professional
You need to get in contact with the Rights of Way team at the County Council, and they will probably ask you for a grid reference of where the signs are.
Another point worth remembering is that a right of way being shown on an OS map doesn't necessarily mean that a right of way legally exists, you have to check something called the definitive map which will be available at the County Council, though most planning authorities have a copy available to look at if you ask really nicely