I've just been on to tech support (again) and they've advised that anyone who does not feel that this recurring problem is being dealt with adequately should write to:
NTL Communications Ltd
PO Box 50
Incidentally, there's 11 outages recorded on their server status page (and that page only contains the major ones!).
Please don't send me or copy any links to the various unofficial NTL websites as I'd rather this is dealt with by genuinely affected people who are having problems rather than a group of people who will just complain because it's NTL.