I have seen a few answers to this on SOF but most of these are concerned with the use of subdomains, of which none have worked for me. The common one being that the use of
For the benefit for anyone else interested in this functionality, there is no simple answer I am afraid. Google "Single Sign On" and it will come back with the technology and some solutions avialable.
As for using htaccess to hide the domain name, this is not possible as it could be used for malicious activities.
I have now successfully implemented a system to achive my requirements.