It's where this map says it is:

http://www.magic.gov.uk/website/magic/opener.htm?startTopic=magicall&chosenLayers=moncIndex&xygridref=391789,206089&startScale=2500

You can't see it from the lane, you have to go through the gate into the triangular field and search through the undergrowth in the corner, just a few feet from the lane.

Baz