The NtlWorld Service Status page lists ongoing "Planned Maintenance 4th Jan to 11th Jan", which occurs between Midnight and 6am. The "optimisation" and "upgrades" scheduled for the 10th and 11th affects "Acton, Cosham, Belfast, Brighton, Stoke and surrounding areas".
I have called them and they have said there is a 'general fault' around London & the South East. They can't tell me how long the service will be down..getting a bit miffed now.