We're operating in an information vacuum - attempting to deduce alignments from a couple of images with no proper georeference data. It's fun, and interesting, but ultimately not very much use.
The image from the 2010 preliminary report at least had OS grid ticks along the margins, the ones overlaid on the Google Earth backdrop I don't trust as much (because I have little confidence in Google Earth's accuracy in the orthorectification of aerial photos).
I do not understand why the project doesn't make available the OS coords of the supposed pits. I look forward to an actual paper being published that might shed some light on this.
Then there's the horizon altitude to consider in each of those directions which could easily affect the angles as pointed out in your comment on Mike's site (sunrise and sunset are not necessarily symmetric about the meridian unless horizon altitude is exactly the same in each direction), the possible viewing locations (at the Heelstone? SE/SW of the Heelstone on each alignment?) and even whether the builders intended a precise or simply a symbolic alignment (the "near enough is good enough" approach).
Finally, I simply don't buy the processional route midpoint theory at all.