SO2631 : The Vision Farm, Capel-y-ffin
Y Fan, across the valley is the highest hill in Monmouthshire. It could be possible that as well as moving the Vision to Olchon, the book could refer to Monmouthshire, which was until 1974 in a sort of dual nationality limbo.
Another, this time quite firm film reference to Monmouthshire being "English" was The man who went up a hill and came down a Mountain" Apart from not knowing that in these parts "Mountain" has a different meaning to most of the English speaking word - It's an area of moorland or rough grazing that is higher than the surrounds, and nothing to do with height, it referred to Garth Hill, outside Cardiff, which was thanks to a tumulus the first 1000' hill encountered as you travelled west into Glamorgan. Which would have been at the time it was set (1917ish) the first 1000' hill in Wales.