Thanks for the quick replies.
Tried it again last night, no On Demand except for anything on BBC iplayer? As you say, very strange.
Powered the box down and then restarted and everything back (for now).
I have not contacted Virgin but have run through the 603 Error help pages withy no joy. Incidentally, live and recorded 4K material through the box from Virgin Ultra and BT Sport Ultimate plays perfectly at all times, as does any 4K content on iplayer.
The TV is a Samsung Q90R (2019) on which all HDMI ports are HDCP2.2 compliant.