From the limited information provided, it appears that your added a zone for domain.co.uk to the ns3.mydomain.co.uk and ns4.mydomain.co.uk name servers with a different IP address. If that is the case, and you queried the name servers, there is a possibility that the information got cached. You may want to try a ping using an external source to isolate the problem.
It seems domain.co.uk resolves to the the intended ns1/2 server, but xxxx.domain.co.uk resolves to the ns3/4 server eg. www,ftp,webdisk,whm etc.
No the servers are not clustered and I have configured how avelinux mentioned.
update: I've just run a ipconfig /flushdns and all seems fine now, even so, have I completed the correct procedure regarding setting up the servers, could I add another server with same domain and create ns5/6 nameservers?
Last edited by bobbyrabbit; 09-09-2009 at 03:39 AM.