You have just been upgraded to genius status!
That was spot on. I hadn't even seen that there was an external mailserver option for the co-domains as well. It's now set up and working just how I need it to be.
You know - I've used my own server management system (which was perfect of course(!), but copyright to my previous employer!) and I've used Cpanel/WHM (urgh). I've not yet found anything of note I can't do in ISPConfig. Why the whole world doesn't use it, I'll never know!
Thanks once again.