Thanks,
As I said, the network settings on the box and router are all good and if the cable was 'dodgy' the box would throw up a network access error.
It is the correct HDMI port and 4k TV is fine through the same cable
In the interest of being thorough, tried in turn :-
Different network cable
Different HDMI cable
Different HDMI ports on TV
Different video output modes on box
WiFi instead of Ethernet
Swapped boxes
Only the last action worked so the fault points to the problem box itself
Forcing another power cycle reset and subsequent configuration reload from the Virgin Media Service mysteriously cured the issue - go figure