It was part of a booked day tour. Drove out of London very early, spent the morning at Stonehenge and the afternoon in Bath. Was a nice day, though Stonehenge was the highlight. There were about 20 people in the group, and the weather was cold, but very clear, so we got several great pictures.
Ah, yeah - some of the tour group companies block book the special access on certain days.
Iirc, about 20 or so is the limit on how many they take (mostly so they can reliably stop people trying to steal bits of stone, used to happen a lot).