First time I saw this Dolmen was on a school trip, perhaps you could hang around and wait for a group of kids and just walk in with them.
Perhaps not,
Last time I came here must've been nearly ten years ago, pre digital camera days, hence no pictures by me. We paid to go in, it was if I remember, a ticket to go round the gardens, rather than the big house as well. You have to go past the Dolmen to get to the gardens, the other side of which is Bryn Yr Hen Bobl, on private land to be sure, but there is but one gate and a hundred yards to the chamber.
Not much help I'm sure as it was decade ago, but that's what we did.
The gardens were nice too.