Just to let everyone know that this problem is now fixed, thanks to the
people at voip.co.uk. The reason was because our local LAN uses IP
addresses in the range 192.1.1.x and these aren't private addresses
apparently.

It's a historic thing and one that's difficult for me to change. So, I've
set the phone to use a its default static address of 192.168.2.2 and
adjusted to router to use 255.0.0.0 as the network mask, to allow it to
route everthing on the network.

Thanks for all your help - it's much appreciated.