Once you have eventually found the place, asked the land-owner, climbed the gate, avoided the cattle and what they leave on the ground, it is well worth the visit.
We visited this site a year or two ago just as the sun was going down. It's in the middle of a grassy field and we did trot down to the nearest farmhouse to ask permission - but it turned out to be converted into holiday cottages. So well we hopped over the gate and ran over.
Just a couple of stones propped up against each other, quite small - but a beautifully pinky colour that just glowed in the sunset, a lovely calm friendly spot.
According to this information from the South Gloucestershire SMR, http://ads.ahds.ac.uk/catalogue/search/fr.cfm?rcn=SGLOSSMR-SG2402
the field in which the stones lie has been variously called Chestles, Chissels and Castles. There used to be a mound associated with the stones, and there were five stones here until about 1760. (It's not immediately clear to me which references given on the webpage refer to which bits of this information).