We went for the weekend a couple of years and did both park and ride and parking on site. Of the two I’d recommend parking on site as it wasn’t prohibitively expensive (if I recall correctly about the same price as the park and ride although I can’t recall exact pricing). For convenience it was much better parking on site and wasn’t particularly difficult getting in or out. We parked in the one just before the main gates on the right (just before the Dunlop bridge).
Sorry my memory isn’t great but definitely on site was far better value and convenience.