Not wanting to jinx things, I've run a complete factory reset due to newly appearing Prime Video errors, and all appears well at the moment. I also had to restart the hub (v3) as network speed went down to atrocious levels for no apparent reason.
Netflix reports software version 4.35_76_20210916060857... (yada yada etc)
UI release 20211123_18925... (yada yada etc)
Box itself now on v4.35 (app 76_2021091)
Sadly doesn't really tell me why the issue started or stopped.
Hmm... well now I'm getting an CS2401 error after going out of Netflix and wanting to go back to what I was watching. Saved programs etc report same issue diagnostics report connection speed is now slow. The box also reports CS9994... There's no issue with our network here as I'm responding on here using the same wifi, hub reports a tested 655mbs downstream and 30mbs up so I'm guessing this is a VM issue and not down to us.