Finally managed to nail the problem I've been having trying to get on to
VATSIM's website, and with ServInfo reading VATSIM data.
NTL's default web proxy cache had gone haywire. I have now reset my various browsers and ServInfo's settings to use an explicit web proxy and I can once again get VATSIM data in ServInfo.
Anyone else on NTL (or Blueyonder) cable broadband experiencing similar problems with certain URLs inexplicably failing to load may want to refer to this help page - http://homepage.ntlworld.com/robin.d.h.walker/cmtips/trancache.html