Update 1 July: The datacentre say that whilst the UPS was disconnected for planned maintenance they incurred a coincidental loss of utility mains power, and thus power to the site was lost.
Since roughly 13:43 it was noticed that a lot of our TT DSL lines were flapping.
After initial investigation we have concluded our physical fibres and BGP sessions have remained up. Therefore we believe the issue to be within TalkTalk's network.
We are chasing TT at the moment.