Okay folks,
I've solved this for me, and am putting the steps I used in here for other users who want to do the same.
1. Provision second server with same version of Zimbra on your main server, with the following items to install. (Please follow directions in multi-server guide also, i.e. make sure time is sync'd on all machines that you are putting zimbra on, and DNS is working)
a) zimbra-core
b) zimbra-store
c) zimbra-apache
d) zimbra-spell
1.5) Do firewall stuff to allow access to mailbox server. (note for me)
2) Using the zcs-to-zcs migration here to export data
ZCS-to-ZCS Migrations » Zimbra :: Blog
3) scp the file to new mailbox server
4) delete the user
zmprov da <user@domain.com>
5) create user on new mailbox server with the following, to take note of adding the zimbraMailHost so it gets created on specific server. Do this on the new mailbox server.
zmprov ca <user@domain.com> password zimbraMailHost <new.mailbox.server> displayName 'First Last'
6) import the mailbox using the link above in 2) (again do this on the new mailbox server)
That seems to be it for me, I now have a way to move old user mailboxes off the main server, and still be able to access from anywhere.
Thanks to everyone for their help and forum posts which enabled me to sort this out.
Regards
-Roy