The OS description is PO N ANG NE FACE. PO is post office, and this is now, "The Old Post Office". The benchmark is supposed to be 0.6m above the ground, but using my walking pole I measured it as 0.86m. The mark has been moved up, presumably when some stone repairs were made. This is an image I took of the surroundings
ST7991 : The Old Post Office, Tresham. If you compare the stonework with
ST7991 : A sharp bend in Tresham, it is indeed the case. The stone on which the benchmark is carved is not visible in the older photo. What's more, this mark seems rather distorted - the limbs of the arrow are not symmetrical. I think it was replaced, rather than just moved, when the repairs were done. It's a pity that the lower section of the wall is not visible in the earlier photo.