Whilst visiting Old Wardour castle (pretty site on the side of a lake) I took the opportunity to have a look at the grotto. There is a car park outside the main gates and a small shop/ticket office on the left. The grotto itself is easily seen on the left as you approach the catle. Facing the grotto, take the small path through the trees to the left and the first thing you see is a fairly large single standing stone - about 5ft high. A little bit further down the path you come to an odd arrangement of stone seat alcoves. Clearly vivble are two much larger standing stones used to help build the seats - very odd. (This is directly behind the shop/ticket office amongst the trees). The E.H. guidebook for the castle states the stones were removed from a circle in nearby Tisbury to help give 'barbarism' to the site!!!!!!!!!!!
Alternative information about the stone(s), mentioned by M Cunnington in her 1949 'Intro to the archaeology of Wiltshire'. She says the following is "all that is known of the circle" and is Hoare quoting William Cunnington (in his An. Wilts I).
In a field near Place Farm, in the parish of Tisbury, was a circular work with a vallum set round with stones, and a large stone placed erect in the centre. On removing the stone (which was twelve feet high and four feet wide) by Lord Arundel's orders, to the old Castle of Wardour, a skeleton was found, at the depth of 18" under the surface, deposited close to the central stone.
Apparently, at the side of the grotto are some stones moved from a prehistoric site at nearby Tisbury, c1792. (Though according to other accounts, I've read, the stones are actually part of the grotto itself.) Josiah Laine, who built the grotto, came from Tisbury, so I guess he was using what he saw to be handy and suitably quaint stones. Supposedly the human remains beneath the stones were placed back under them at their new location. You wouldn't imagine they'd be best pleased at becoming part of a fashionable garden accessory.
The stones are thought to have come from a neolithic tomb at Place Farm (ST951299) in one of three fields near the junction of the Chicksgrove and Chilmark roads. (http://www.wiltshire.gov.uk/community/getcom2.php?id=225)