Redhorn Hill, now there's a thought. Suppose instead of avoiding hills on the route from the Downs to Salisbury plain Neolithic man used these hills in the same way that modern man uses the moon when sending satellites into deep space.
Rowing a heavy stone up a steep hill is certainly a slow process, however having reached the top of the hill, Neolithic man would have achieved much much more than just the distance covered. For every inch of altitude gained he would have stored up useable energy. This energy could then be employed to send the stone, not just to the bottom of the hill, but perhaps way beyond.
Row uphill, at the top of the hill slip rollers underneath and lay a path of roller's all the way down and beyond, then send the stone on it's way, imagine the momentum forty tons of stone rolling downhill would create.