I spent more time than I expected installing a new router (or router as I normally pronounce it). I'd expected a quick plug and play but forgot I'd have to add ISP and internet settings. And it's always the little one that you'd expect to make no difference that makes the difference. In this case it seemed to be the setting dictating time before disconnecting. Strange.
Anyway, it's working now.
Next thing is to work out whether the old one was actually broken or not. It was definitely a bit intermittent and would often just stop working for a few seconds. And so far (touchwood) the new one isn't doing that. I think it was probably on its last legs. And worse case is that I now have a spare emergency router. For emegencies.