Reputation managment fails
BT normally trawl forums doing their utmost to remove negative comments. Simple facts with BT. You are dealing with penny pinching, lazy morons. It's almost certain any manhole chamber was built by a third party contractor for the lowest possible fee.
That said, and for all their ills, it's unlikely they would be negligent to this extent. How about National Grid moleploughed that main in forcing it under the chamber when they struck an obstruction?
Just a thought......