Welcome to the Tiled forum, @Richard_JE_Cooke!
There’s a tool bar with some small icons below the tileset view:
This tool bar includes “Replace Tileset” and “Remove Tileset” actions (the two right-most icons). If the “Remove Tileset” action is disabled, then the tileset is already not part of the current map, but it means the tileset is either part of another opened map or opened by itself. To remove a tileset from the Tilesets view, the tileset should not be open by itself and it should be removed from all open maps.