I found another stone with a large Ammonite on it near Avebury. The site is the Shelving Stones, which according to archaeolists, was destroyed 200 years ago.
The stone is now in someones garden as an ornament.
PeteG
Ps. Spectacular Panoramas of Stoney Littleton here http://web.ukonline.co.uk/megalithics/england/stoneylt/stltpjs.htm