I basically followed the 123-reg instructions (eventually).

If I remember correctlY:

Having established the hosting, from the existing site from e-voice Website Settings.Site Domain  I made a note of the IP address which I needed to point the domain and then went into the Control Panel in 123, unlocked the additional domain name then went into Advanced DNS and put in the destination IP.

If that isn't quite right just have a play around, it took me down  quite a few dead end but got there eventually.