Thanks for your response, we do appreciate all the hard work you guys are putting in! Sorry for hammering but sometimes it is hard to explain to a client why we now can't do something that we said we could. They wanted to see that we were doing all we could to fix the issue so your response was appreciated.
We have managed to find a satisfactory workaround for now so that we can go live!
For anyone else with this problem, we created a separate login page for each plan. Users are then emailed their specific login page depending on the plan they selected. We then used a YOOlogin module on each login page and specified the desired login URL for each. This bypasses the CB/CBSubs settings and therefore bypasses the problem.
Happy days!
In all seriousness, we are keen to join the test team if you want us. We feel we have more to give to both CB and CBSubs and would love to get more involved with the projects.