Thanks Moss, no I hadn't read that before. I see Dan Hicks uses the term "Outstanding Universal Value" but doesn't try to quantify it in any way.
He is helpfully clear in his explanation of English Heritage separating into Historic England and the English Heritage, the latter having charitable status. And hey ho! Highways England, a government owned company, set up more or less at the same time. What a coincidence.

To quote:
"Some things are plain to see. The Stonehenge Bypass plans put the World Heritage Site in danger. Any potential benefits to the National Trust Stonehenge Estate do not justify the damage to the rest of the WHS. It is impossible to ‘reinstate’ the route of the Avenue. And the negative impact of the current A303 comes not from the historic road, but from its 21st-century traffic."
If there must be a new road, I think the suggestion to "loop the route of a new bypass wholly outside the boundaries of the WHS" is the way forward.