The ISP may be relevant. On Tuesday, I suddenly couldn't access any websites, although my broadband link was connected and logged in.
After some investigation, I found that I could access sites using their IP addresses. Seems UKOnline suddenly and without notification no longer supported Server Assigned DNS (Domain Name Servers). I had to get the IP addresses of the DNSs from UKOnline and configure manually.
I am still awaiting UKOnline's explanation as to why they have done this.