Nettle, SSH and public keys

When I log onto one of my servers for the first time using SSH from another Linux machine I get a prompt about accepting the key and it then uses this to populate the ~/.ssh/known_hosts file.

If I try to log on to the same machine using Nettle I get an error:

Nettle: SSH connection failed in step 0: unable to exchange encryption keys

It seems that !Nettle connects fine from my main RISC OS computer but doesn’t from RPCEmu…I guess this is down to a networking/configuration issue with RPCEmu? Possibly due to the IP tunneling?