I booked twice as soon as stu posted the info on here.
When I was there on Saturday, I did the cave art tour at noon, then the 'normal' tour at 1:30pm. Just as the 1:30pm 'normal' tour was about to start, someone came in and said that four people had failed to turn up for the cave art tour and would anyone like to take their places. If you'd really like to go, then I suggest you ring them up and see if this happens often.