Another theory I saw is that the Broadstone could be part of a Cromlech, (may be a capstone?)
ref : The King's England- DORSET by Arthur Mee, page 303)

That would explain the 2 other stones nearby. I saw a copy of TMA book that showed the stone with the Drude himself sat on it.

It looked fairly visable at that time. He mentioned that in recent times the stone was erect but had to be lowered due to to the number of cars that ran into it!!

Phil