It's well worth going if you are in the area. Try and go on a weekday morning a few weeks into the summer holidays. I went on a tour designed for 20 with only 4 others and the guide. That meant we could all go in and see the rock art at once so had much longer perusing it (normally you have to wait outside the cave whilst others have a look due to lack of space).

Alternatively contact UBSS (Bristol Uni Spelealogical Society) about visiting Aveline's Hole onthe mendips as they've found some there...