The screens behind show BBC One digital - only analogue was affected earlier this evening (see other thread)
Not strictly true. They show the output from DNC1 directly. There is no monitoring off-air of BBC digital channels unless specifically requested at TV Centre.
In other words, DNC1 could be broadcasting quite happily, but the link to NTL could be broken so you the viewers might not be seeing anything and DNC1 is oblivious to the fact that they aren't radiating