Hi, till, thanks for taking the trouble reply on a Sunday evening. I appreciate that.
It turns out there's no need to do it that way. If you go into Management -> Server/Settings, on the first screen, you can change the server name to what you want it to be from Server1 and you can change to any domain you have configured in Domain:
This allows you to go to https://mydomain.com:81
and your ISPConfig login will show. This is the address you can give to your resellers and users. The other one still works, but the chances of anyone else stumbling across it without knowing your account details as given by the server's or VS's host are remote.