I heard about this stone two or three years ago via Terence Meaden author of The Secrets of the Avebury Stones. Yesterday, with difficulty (and help from friend) I found the well hidden Standing Stone at Stanton Fitzwarren at SU182903 (Explorer 169). Definitely a sarsen, it stands the same height as myself at 5'4".
Knowledgeable friend thinks it may be a boundary stone; it is true that there are many large sarsen stones in the hedgerows around Avebury. Stanton Fitzwarren is north and not typical of the Wiltshire landscape, just off the A361 between Swindon and Highworth. No sarsens in this terrain at all and this stone does not look like an 'imported' one that can be found around Swindon. The hedgerow seems to have been planted around it; it is off the beaten track on the edge of this ancient village (manor house, church and mill).
Would be grateful for any information.