For future reference should someone search this thread - my (home) network is "non trivial" with separate routers for internet access (DHCP / DNS - default 10.1.1.1) and wireless (default 192.168.x.x). It is probably possible to configure my network to work, but I was keen to actually use CC (!) so I switched on the wireless on my internet router - and connected both directly to that - and that worked.
As an aside, the Calibre -> Preferences -> Sharing over the net -> Test Server function looks useful. It is invoking a browser (with 127.0.0.1:8080 IP ??).
AND that page provides Server & Client access logs. When I have some time I will look through them.