Seriously though, how has this escaped everyone's notice before!?
The Ordnance Survey has marked it as a physical feature. It is marked by a black circle-ish shape in the field. Not identified as a "Stone Circle" though.
That black circle feature has appeared on the OS post-1967. The patch of rough ground where the circle lies (within the improved field) is shown as a seperate enclosed marshy area on the OS 25 Inch 1897-1914 sheet.
The fact that it only appeared post-1967 suggests that it may have been picked up from aerial photos rather than fieldwork surveying. As you said earlier in the thread, quite possibly when the OS actual went there to survey for the County Series 25" it may have been hidden in a peat bog.