Zhadnost - thanks for checking. This would be my first guess also, but I have ruled it out as I have 7 US private proxies across different providers, IP classes, and different locations/NOC's. They are all working correctly when tested by freinds on other ISP's
From your post, the block seems to be limited to my connection (but I was never suggesting this was a Virgin wide issue).
I wonder could it be something on my own router thats causing the problem (restarted it multiple times - no joy).