Thanks omegatron, I figured it out eventually - unfortunately, the solution was not one I wanted to hear.
I had to clear all my cookies, plus wipe my saved passwords from autocomplete and then I got access to photopost instead of photopost classifieds.
Of course, I then had to do a password retrieve through vBulletin to come here and tell you about it as all my cookies are now wiped. I now have to go to all my regular haunts to reset all my passwords.
I'm sorry guys, but this is without doubt, the most ridiculous license management system I've ever encountered.
Nice gallery software, but you get a Darwin award for the license management.