Thanks again Till, I wasn't aware of that tab :-)
Now I have another problem though - having made that change.
I have 10 clients who use theirname.main-domain.com and I've set up the site to have theirname as the host and main-domain.com as the domain name.
There are now many entries in isp_isp_domain with host NULL and domain main-domain.com. I've also noticed that if these clients wish to create an email address for themselves, it's set as @main-domain.com, not @theirname.main-domain.com
I presume that I have to put theirname.main-domain.com as the domain, and leave the hostname blank?
I'll try it and see what happens, but would like to know what the 'official' way os to set these up.