In reply to Phwee:
Boulder Britain has some info if you can get a copy of it.
Failing that:
the Snoopy boulder is on your left immediately next to the road/track between the first parking area and the second parking near the castle;
the Pickled Walnut boulder is further down the hill and slightly left, following a path from the second parking down in the direction of the town;
the Classic Aretes boulder (which is probably the best area for a first visit) is up the hill (from the first parking) past the big monument, then down the other side to your right; and
the Pond Wall boulder is just above the Classic Aretes area.
That should give you enough for a couple of visits.