I think dinas rock is just about in the brecon beacons national park boundaries. It can be dirty as it doesn't seem to be that poplar these days, so take a brush. The trench near ogmore is the best place to go bouldering in S.E Wales though, all of the sandstone bouldering being properly sh1t. Think you can get topos, directions for both the trench and dinas rock off the south wales bouldering site, given by the link above. Hope this helps.