I didn't know about this when we visited, nor the actual car park; we parked in a short layby direct on the A4 (a bit of an interesting experience crossing the road), but it was to visit both Silbury Hill and the West Kennet longbarrow. What I didn't realise at the time was screened within the trees beside the pathway to the longbarrow there is another Hill, not quite as tall.

It's a fascinating area.