[eUK-Nick]

You first need to setup the nameservers for the sub-domain.

Once done, you would have to open the db.domain zone file on the name-server (this is for the main site).

In there, first - add a nameserver resource record for each NS of the subdomain.

Then put in "A" records for the NS listed in the nameserver's records.

Once you have done this restart the name server with /usr/sbin/sig_named restart

This would reload the NAMED databases.