so is BT responsible for the broadband or not?[/QUOTE
yes BT provides broadband through exchanges which they own
but the Corporate side of BT is very different. Company's use them and pay lots of money for very expensive contracts for comms.
When your link between your server farm and all your branches gets severed, and the whole business grinds to a halt, IT managers tend to get very irate. And as these IT managers spend lots of money with big money contracts, they tend to take notice.
BT very rarely has problems on a user level. It will be all or nothing. So when your site link goes, everybody's will more than likely go. Because of the money involved, BT will take much more interest in it than they will for broadband customers paying £20 a month or whatever, when corporate customers are contributing tens of thousands per month.