Good news. The problem has been fixed.

I just went through the router settings fiddling. I noticed that the DHCP settings was set to “manually configure DNS” and when I changed this to automatically, it started to work properly. I had also set the TV back to DHCP. The DNS is set on the TV to the default gateway. The DNS on the router is set to 8.8.8.8/8.8.4.4.

I have no idea why this resulted in the behaviour it did.

Thanks for everyone's help

Jim