Devil's Den was the remains of a long barrow - the mound destroyed and the stones fallen. It was restored in the early part of the 20th century by a well meaning antiquary who restored to how he thought it should look. I was puzzled as to why it was the only 'portal tomb' in Wiltshire until reading the superb little book by Peter Fowler and Ian Blackwell An English Countryside Restored - The Land of Lettice Sweetapple (found in my local Tourist Information Centre and still one of the most useful reference books on the Fyfield/Overton area there is).