App store not working

Having problems with the app store on my primary mac pro. It won't download any updates or install Lion. No problem with the account as have installed Lion on alternate Mac Pro, MBP and imac.
Basically I just get a spinning wheel in the bar at the top.
I have tried, deleting preferences for app store, logging out in app store and itunes, removing system drive from spotlight and then re-adding, repair drive preferences etc.
App store recognises I need updates but never actually starts downloading them when I click update. Similarly it won't start to download lion.
I have had problems with the app store on this machine for a while but I think it has been intermitent. Now it won't do anything.
Anyone got thoughts on what to do?

Just tried this and it does exactly the same.
I have to enter my password, then all I get is a spinning wheel in the top left next to the fwd/back buttons. No flying icon into the app folder.
Any other thought anyone?