THERE is nothing quite so challenging as the stone circles, megaliths and other standing stones that grace Britain. Stonehenge, Wiltshire, is of course the most famous but others have a more magical quality. Avebury stone circle, near Stonehenge, is less frequented by tourists but is much more atmospheric than its more famous neighbour. It's humbling to stand in front of these structures and know that man built them for reasons unknown thousands of years ago and still they remain. A "message in a bottle" sent through time for us to deciper.