There is a BBC article that details the route to the top of Garth Hill.
Useful link for OS maps:
Open Data Viewer (or, in simple terms, a map (one that you can search by OS reference )
Grid Reference Finder (this is great because as well as searching by grid ref you can right click on map and it will give you the grid ref!)
So, there is Llancayo house! SO 36770 02979 (simple terms SO 36 02 – area SO 36 across, 2 up)