A secondary server I have simply refuses to pull its zones from the primary unless this is set. Our hosting controller doesnt set this, so it makes automated secondary DNS impossible.

Both NS servers are NAT'ed and are setup to use each others internal IP's - but this is still not working.

What Control Panel are you using ?
There is a feature of automated updation of Dns Zone from
Primary Dns to Secondary Dns in " Hosting Control Panel".
Hosting Control Panel is only distributed Control Panel.

<<Removed contact details/this isn't an advertising forum>>

I'm using Hosting Controller. It's not a fault of the controller software, it is something to do with MS DNS behind the firewall on NAT IP's I believe. Even with debug logs we have been unable to get the DNS to propagate. I was just looking for a quick fix :)