Thanks for all that, I finally fixed it. It wasn't your suggestions that were the problem (they were all fine and pretty much what I was using). I'll give you the boring details of my thought process (such as it was).....
Hmm, thinks I, Tbird is OK but OE stuffs up = Some sort of IE/Windows problem.
Try using IE instead of Firefox to access https stuff on server = Fails!
Thinky, thinky, add beer, thinky....... !ding! some sort of encryption/cert problem.
Download+Install windows 128 bit encryption...reboot....
Try IE/https = success!!!
Try OE+IMAP+SSL = success!!!!!!!!!!!
Only taken me about 3 days
Still, useful thing to add to the howto, ALWAYS check IE FIRST (help->about cipher strength = 128bit) else it's unhelpful error messages all the way.