Theres an OS sketch here:https://www.ordnancesurvey.co.uk/gps/Stationpics/gif/C1NS8876.gif
The sketch distance 'A' of 1.27m from a fence post looks promising and is consistent with
the measurement taken by Rob in 2014.
You might try a long-bladed screwdriver rather than a fork. Not so good for the knees but
if you check at intervals of, say, 6 ins then its easier to distinguish between buried stones