I've just had a call from NTL regarding the Poplar proxies, they guy told me that Valve are currently blocking the Leeds and Poplar Caches from accessing steampowered.com, and that it should be sorted in a day or so (not sure how that affects the Hersham/Colchester etc ones but anyway I guess its all related somehow).
Presumably someone has been a naughty boy (or girl) in those areas
Blimey, Cogent and NTL contacting me in one day...I must go and lie down..