We visited there when our kids were young, when we got there, the front car park was full, so we had to park in the side car park off to the left.
At the bottom of this area was what I thought was another entrance, under a bridge, so we entered the park through there. We entered through a wooden swing gate and looked around for where we had to pay! close by was a couple of security guards, I asked one of them "where do we pay?", he replied "You're in now, but It's round the front, if you really want to pay!"
Anyhow, we spent the whole day there for free!, but I have to say that I was uneasy the whole time we were there, expecting somebody to confront us and ask for our passes!