That's a good thought actually, I hadn't spotted that.
Still no joy though.
If I add even just the site to ISPConfig, even without creating a user, even with that option selected, then it adds the domain to the local-domain-names file in /etc/postfix, whether I like it or not!
That, of course, means that any mail being delivered locally from the machine to that domain (like, from me for example!) will be incorrectly routed.
Any other ideas?