So, I needed to copy my vCenter Server Appliance over to another host. Obviously can't do that while it's switched on, and it takes care of the migrations in vSphere Client so what can we do? Download ovftool (I downloaded the Windows version) from http://www.vmware.com/support/developer/ovf/ and install it on your PC. Go to the installation directory and shift-right-click to choose 'Open a command window here'. Type the following (note it is all one line):

ovftool -ds="Destination Datastore Name" "vi://[email protected]/VM Name" "vi://[email protected]"

Don't make the mistake I did of including the VM name at the end of the destination. This is so quick and easy and requires no copying of an OVF template to your PC then back again, it does it straight between the hosts. Hope this helps!