So I asked the Department for Culture, Media and Sport, as the successor department incorporating "National Heritage", if they could inform me of the procedure to inspect the said plan as the regulations depend on being able to do so to determine the area of land covered by the regulations.
They sent a copy of the plan:
Click to enlarge
Link to map overlain on Google's aerial photo which shows that the old A344 isn't included:
From Simon's comment below
They don't know, it is lost; "I can confirm that the Department does not hold the original plan. You may wish to contact Historic England or English Heritage to find the information you are seeking." was the reply.
Without the plan the regulations would appear to be useless. Accused of transgressing them a defence would be "show me the plan to prove I was within the area". The monument itself is protected under other laws anyway.
And of course the regulations have no penalty attached so are merely a civil matter anyway.