[Spencer]

So if the whole two muxes are being fed by satellite to the Bilsdale relays, what’s the purpose of the solo BBC One NE&C HD stream reported by @harshy and the SD equivalent mentioned by @Stooky Bill?

I don't think it's a seperate 'stream', they're the BBC One's that have always been on 27.5w, but just fed with the North East version of BBC One 

The transponder carries seperate services for the channels on PSB1 and PSB3 including national variants of BBC1 (see below) and the transmitter selects the correct combination to transmit. 

Thanks for the info and explanation - really interesting.