I bought some land 40 years ago which had a disused public footpath running across it. I built a bungalow on the land, but a few years ago the council told me I had no right to block the footpath, even though it runs through my garden. They now want £450 to divert a disused footpath.
A footpath cannot be lost because it is no longer used: it remains a footpath until it is closed by an order made under statute the type of order in fact that will be required to divert the footpath round your garden. There are cases where people have to put up with members of the public walking through their gardens, so the council are accommodating you in agreeing to divert the footpath.