It's just that I have seen mobility scooters there before. In fact the last time it was the 'keeper' himself. So I didn't realize there was a problem.
From what I can find out. Blue Badge holders can park for free in the Main Car Park, the Village Car Park, and in the designated spaces in the Old Farmyard and conditions at Avebury are good enough for mobility scooters to access most of the areas. No need for a tarmacked car park then.