The National Trust car park is nearly always at least partially full between the times of 10am - 4.00pm (later in the summer months of course). Anyone who wants Avebury to themselves should go a bit earlier/later or walk out beyond the stone circle in almost any direction to experience solitude and few, if any, other people. As you know Moss, the Avebury World Heritage Site encompasses a wide area of great archaeological wealth plus a profusion of wild flowers, butterflies etc. I personally think it would be a great shame if visitors were charged to go into the circle in the name of preserving it - hard to enforce too (except for gullible tourists being shepherded around).