Oh dear, I really didn't want to comment on this topic. I know the site well as I travel to Avebury on a double-decker bus and get a good view.
A couple of points that haven't been mentioned. And I'm just playing devil's advocate here - I agree in principal with the stance HA have taken.
There are already buildings right next to the site.
The site is small so no way are five luxury homes going to fit there - I would envisage a terrace of small homes.
As HA feel very strongly about this matter, with justification ..... how about starting a campaign - in honour of Alexander Keiller - to raise the money to buy the land fom the developers (they probably want to get shot of it anyway) and then give the land back to the nation in trust so it can never be built on. I am sure if you broached it in the right way the National Trust and other local organisations would support you.
Just a thought .... no provocation intended.