Do they though? As I said, I walk there regularly and in over a hundred visits I have never had to pay for access. I do give into the visitors centre 'to use the facilities' but have given up on using the restaurant there for a quick coffee (overpriced) and have not entered the exhibition area in the centre. In fairness, I do only pass through the area around the centre and avoid it as much as possible and haven't used the official entry to the area (and the overpriced parking area across the road) for years.