I can connect using filezilla without any problems - it's just command line that is giving me the problems.

I don't know if the error I'm getting is from my sshd service or the clients' sftp server?

If it's the clients server then what do I need to ask them to do to accept shell commands? If it's my end then what can I do?

It seems that I can connect to the client and be verified using the key they supplied, but then loose connection when I try to communicate :-(