Local host names on TalkTalk internet
TalkTalk implements an Error Replacement Service, redirecting non-existent hosts to their own server via DNS. Who knows why? One of the deleterious effects is that although the router (I have the new Sagemcom clutch bag) collects DHCP names, DNS lookups for those names don't work.
Opting out of ERS doesn't seem to work, or doesn't work immediately, even on rebooting the router.
The solution is to go to the router's advanced configuration page for the internet connection: Dashboard>See Internet Settings>Manage advanced settings>Internet Connectivity (or some other route through the maze of twisty little passages). Choose Manually Specify DNS, then put in the addresses of the Google DNS: 126.96.36.199 and 188.8.131.52. Click on Apply.