This is an intermittent problem. I have been through Apple Support who advise that the problem is with ISP. Network settings tell me "The cable for Ethernet is connected but your computer does not have an IP address" or "Ethernet has a self-assigned IP address and will not be able to connect to the internet" depending on which location I use. Everything has worked very well since 2019. If I disconnect the router from the back and re-plug it then the computer connects OK. Do I need a new router? How do I get in touch with Now TV directly re this issue??
Well unplugging was effective and the only way I could connect when the problem arose. However (for other readers with this issue) apparently this does not re-set the router, for which you have to press the tiny re-set button on the back which I had not noticed. Otherwise the next step is a replacement router (and maybe replacement specs).