Simon Banton's excellent site is getting better and better all the time -- it is now really very informative, and I hope that it will attract comments from others who hold information about particular stones. Congratulations to him, and thanks for all the hard work!
Simon has added a comment on the missing stones and the parchmarks:
Four sarsen stones that might be expected to complete the sarsen circle are also missing (eg Stones 13, 17, 18 and 20) and so those stones do not have pages either. Evidence that the sarsen circle was intended to be complete arose during the dry summer of 2013 when a series of parch marks appeared in the circuit exactly where some of the missing uprights should be.
Simon Banton, Mark Bowden, Tim Daw, Damian Grady and Sharon Soutar (2014). Parchmarks at Stonehenge, July 2013. Antiquity, 88, pp 733-739. doi:10.1017/S0003598X00050651
I note that he uses the words "evidence that the sarsen circle was intended to be complete......." rather than saying "evidence that the sarsen circle was complete......." I'm happy with that, since as I have said before, the fact that there may have been pits in the places designated does not actually mean that they ever held stones -- and so the evidence cannot be taken to show that Stonehenge was once a complete monument.