I'll be honest chaps and say that I have never had an issue logging into the site at all using IE, this has been at home and at work. Strange as I use the same Internet proxy at work as Ben.
The only issue is that its slow at times, but nothing like the MRC site.
Once membership levels get a bit higher we will be moving to its own dedicated server - that will solve the speed issues (at the moment it is sharing with a couple of very busy websites)
I believe that the issue is related to cookies and those that joined the site early on have cached the old cookie - clearing cookies may solve the problem (it worked for me). This should not affect later users.