You're right RG. People have been reinterpreting and reusing the sites since they were created. Stonehenge is probably a case in point - surely it didn't mean the same thing to the people using it over its development over 100s (1000s) of years?
People have reused barrows from earlier times, included them in their new forts or monuments, gone to them for contact with the supernatural. Or even nicked the stone because they didn't care, or precisely because they were showing they were the boss now. It's symbolism. You can't just ignore it or you really are getting further away from the so-called Truth.
British Archaeology have a relevant little article at
http://www.britarch.ac.uk/ba/ba29/ba29feat.html

As Howard Williams, the author of the piece says:
"This long tradition of ritual activity at ancient monuments represents a strand of continuity in social life in Britain, across the Roman–medieval transition, when so many other practices underwent drastic change. "