I can not demote it because replication connectors don't recognise the RPC. Articles I've read imply that the dual adapters confuse the DNS, but offer no solution.
Can I just remove one or the cards or will that make it worse?
Problem was installing Small Business Server, which can be the ONLY domain controller on the domain. Removed the firewall on the adapter and removed the second (dynamic) DNS entry, created a manual entry for the Sites & Services replication connector, forced a refresh of the topography and once the replication was complete was able to demote the SBS from the domain.
Started again with Server 2003 R2.