A quick update on access: my old OS map shows the circle as being outside the access land area, so I went and asked in the farmyard (three people, one barking dog) where permission and directions were readily given.
I do note however that the latest editions of the OS 1:250000 map available online show the site as being within the open access land boundary now, so no permission is technically needed. Given its proximity to the farm and the fact the easy and obvious route to it is through the farmyard, asking still seems like a good idea and may be appreciated (if anyone is around to ask, other than the dog).