Having looked into this I have come to the following conclusion.
Mailsync only works 'with ssh' to a remote IMAP server, I'm pretty sure it can't access an ordinary mbox (or other) hierarchy on a remote machine by using ssh.
I *think* the example in the examples documentation is just that, accessing a remote mh mailbox system which happens to have an IMAP server using the mh hierarchy.
If anyone can point me to some documentation and/or help suggesting otherwise then I'd be very pleased to know about it.