A bit more information. Following another Windows update I had exactly the same problem. This time the DNS addresses were still set so it wasn't related to them. I discovered it was the firewall - switch it off and it worked, on and it didn't, and I could get consistent results.
I'm using Norton 360 (which came with the laptop and I don't like much) and the only config I could find was to do a firewall reset. Other firewalls I've used have popped up a message saying 'program changed do you want to allow it' which is clear and simple. I don't fully understand what a reset is doing - is it now allowing everything through without my permission? I'm not keen on Norton, I'm going to bin it once my free subscription expires.
So I don't think the DNS changes were necessary, I think it was the firewall all along, although I'm not sure why it started working last time - I couldn't get reliable symptoms to properly diagnose.