"a date of 750AD plus or minus 110 years on the Senegambian stone circles "as a result of Laboratory tests at the University of Dakar". Disappointing. I'd like more information on the tests before giving up on the stone circles being prehistoric"

Depends what you mean by prehistoric. Whose history and whose prehistory? History didn't start in 1 AD all over the world. If the Senegambian stones were erected before THEIR written history began, then they are prehistoric and quite independent of our dating system.

eg. 3000 BC is regarded as prehistoric in Britain but was historic in Sumer, Egypt, China and India because they kept written historic records.