[Stooky Bill]

It did make it on air during the power cut that took the Six off air, the Dad's Army and IIRC railway documentary that went out came from there. (Although only on analogue and the digital pres areas at TVC stayed online)

Deejay explained how it all worked on the blue place www.tvforum.co.uk 

Yes, that's a good description of it, fairly primitive compared with today's standards. 

It was only ever BBC1 and 2 analogue that it was intended to replace, the digital channels were seen to be dispensible in such a scenario. Although I imagine if it was in use for a while it could have been patched into digital BBC1 and 2 too

The power cut incident seemed to be just a couple of tapes being played to air rather than a full setup, it being over by the time the guys made it up from London. 

On that evening all the pres areas in TV Centre stayed powered, the switch over to the DR set up was more a precaution. IIRC it was only for about 30 - 45 mins and a lot of the country saw their regional news on BBC1 instead of it