Hi Michael, Unfortunately I don't know where to get `Bocking 14' comfrey or even if it's available in New Zealand.
I haven't heard of comfrey spreading greatly by seed however, in my experience it's more that the roots are impossible to dig out once established. So I would go for the russian comfrey if not S. officinalis.
I've just dug up comfrey from a friend's place so I'm not totally sure what kind it is.
Hope that helps- if you locate any Bocking 14, let me know!