More info ... I have done a LOT of googling and found people with similar issues but not seemingly the same cause.
I switched my router back into modem mode, connected the phone line to the ADSL port and kodi started working through the ethernet connection.
Changed nothing other than putting the router back into wifi router mode with a PPPOE connection through my bridged modem and it stopped working again.
Some things I noticed.
Through Ethernet connection when "broken" using bridged modem
from the kodi box, I can ping my router, my PC and the NAS but I cannot ping google.com
Through Wifi connection when connected through bridged modem
from the kodi box, I can ping my router, my PC and the NAS and also ping google.com
Through Ethernet connection when "working" using integrated modem/router
from the kodi box, I can ping my router, my PC and the NAS and also ping google.com
I was using address reservation in my router so that I'd have known IPs for my three Kodi devices, I have removed these (based on a suggestion from elsewhere) and it did not help.
I have noticed that when in the bridged configuration that on the Network status page although it is showing the IP address etc the MAC address is showing as "Busy"