[Stooky Bill]

With the north continuity between 1900-2300, there is I believe 2 BBC1 network feeds. Main network, and North network, which both actually take Spirtsday in each local window (except breakfast which is London), and Sunday Politics which varies 

Is that still the case? If not, what would now be seen if nobody opted out for local news?

Also, when the SD feeds close in 8 January, there will still exist a network BBC1. There has to be. In the 1989s, South East, London and Oxford gitvthe sustaining service with Newsroom South East 

If bbc1 network ceased to exist that would be odd given its never nit existed - if you see what I mean.

Yes there's still a 'network' or 'sustaining' BBC1, and as you say there's actually two as there's a north one too. 

What they carry when I can't remember. It's still London (or South East) for Breakfast and a selected region for Politics/current affairs. It always used to be the news channel for the news opt outs, it might be London sometimes since they moved into the national studio (that might have been a temporary thing)

If a region doesn't opt or fails during it then you'll see whatever's on that sustaining network version. However, every TV and radio service goes through coding and mux and they can replace any region with another. So if a region  can't do an opt and they know in advance they can have another one nearby go out in it's place. Handy functionality, especially during covid. 

They also have the ability to join regions, so if a programme is going out in several they don't all have to be crewed just to put it out

So playout put out 4 (I think) versions of BBC1 - sustaining, sustaining north, SD nightlight, and HD. Yes as far as I know the old HD version with the red caption still exists, but only appears on iPlayer.