You say there is no problem picking up BBCA mux from MyP. This transmitter has already completed switchover on this channel which is 45 and increased power to 16KW. It should also be on the new 64QAM 8K mode so that can't be your problem.
It's possible, I suppose, that the Mux 45 is still held in your tuner. Did you do a complete rescan of channels with no aerial connected to clear out the old tuner data before you did a manual scan?
Note also that MyP is due to complete the dso tomorrow, 25th. November.
Freeview on Moel-Y-Parc TV transmitter | ukfree.tv - independent digital TV + switchover advice, since 2002
Is BBC Northwest listed on your equipment on Channel 800 and 801?
You could also, after clearing out old data from the tuner, try an autoscan leaving out the aerial until approx passing channel 50 then reinsert the aerial.