Solved! For now but don't know why!
I can make a Fritz!Box to Fritz!Box LAN to LAN VPN connection only when using the IP address of each connection and not a fully qualified domain name.
are to use a dynamic DNS domain or one of their own using the myfritz.net setup. I was also told I could use my own FQDN. That didn't work for me and my suggestion is some kind of DNS lookup failure by the router. I didn't try using their dynamic address.
I know my FQDN is correct as a lookup returns the correct IP address of each network connection and I can make a single user VPN connection using it.
I'd rather be using the domain name address as it makes it easier to see which connection is which - if anyone has any ideas!