The circle on the serpent sketch is bisected by the Roman road. It seems likely that if it was meant to depict a disk barrow he'd have shown it exactly in the right place as he had the road to relate it to. So it can't be showing your mound which is a good few feet to the South.

The Roman road at this point seems to correspond with the Roman road, so it seems reasonable that it was cut by the Roman road and then destroyed by the modern road.

One further delicious speculation - the Roman road seems to have an unnecessary Northwards kink in it, whereas a straight line would take it straight through your mound! Sounds familiar!....