After a frustrating six days without a phoneline I decided to reconnect the Grandstream VoIP phone. To my great surprise it worked, found an IP address, connected to VoIPtalk and registered with them. We can both make and receive calls.
So this is now a real mystery. After an evening's fiddling with the settings I'd got the new Siemens phone working perfectly. It went down when Clear Comms changed the login system and hasn't worked since.
Now the old phone works so there must be something in the settings for the new phone that needs changing again but exactly what I have no idea. I'm wading in a sea of stun servers, outbound proxies and voice codecs.