Thanks for the reply. Will certainly look at the links, sounds like some good stuff. Always wanted to get into Linux but never really had a good enough reason but for a firewall i'm almost not thinking twice

There was a problem with the DNS but seems to have now been completely resolved, I had to make sure that the server's DNS IP's were registerd on both NIC's as pointing to themselves and then the DNS server would take care of forwarding the requests, then flushed the DNS cache. After that the DHCP server turned out to be a weird one to sort out. The fix posted above doesn't always actually correct the problem by just ticking or unticking the box mentioned. I had to take the DHCP server offline & remove the NAT configuration from RRAS, then reset the DHCP, reconfigure the NAT and now it seems to be ok

However this has unlocked yet another problem! Which I may post in a different topic but, the Domain GPO cannot be accessed even after adding permissions to all files, delegating control & even booting up in safe mode.
Maybe something got seriously messed up when moving to AD but wanting to be able to sort things out without reinstalling the entire OS.