I think there's probably quite a bit of artistic license been going on over the years.
Looking at Borlase's sketch, I'd assumed the two stones marked 'a' and 'b' were the same ones that Stukeley has as the entrance. But the number of stones is wrong for that, and neither of these match with Blight's or Cotton's drawings. Colour me baffled.

Of course it would probably help if I'd actually been there...