Hang on, let me reboot my core router in case it's having trouble...

Done. Might be worth trying again.

Tracert from here as before ends with

15 60 ms 69 ms 53 ms 81.2.78.28
16 58 ms 60 ms 70 ms warpfield.dionic.net [81.2.78.46]
17 * * * Request timed out.
18 * * * Request timed out.
19 * * * Request timed out.
20 * * * Request timed out.
21 *

I note that I and Dennis, who had problems, are behind routers
(presumably both NAT routers) while Rod, who didn't. isn't. Have you
configured your router to require more than the usual? (Please note the
careful drafting to conceal the fact that my ignorance of TCP/IP is
near-immaculate.)



--
Robin