What would I do in the circumstances you describe? Speaking personally (and this is not, and is no substitute for, professional advice), immediately after the incident in the cafe I would have written a letter to the MB saying that her behaviour in the cafe fell so far short of that expected from an employer that I consider it a fundamental breach of my contract of employment which I therefore consider to be terminated immediately. In addition, her bullying behaviour during the previous weeks caused me to hand in my notice which I have now been advised amounts to a constructive and unfair dismissal by her.
Accordingly, I wish to receive my outstanding pay for the period of [day after last pay period] to today and pay in lieu of the outstanding period of notice from today until [end of notice period]. I also wish to appeal against my unfair dismissal; please arrange for this appeal to be heard and forward me the details.
But as I didn't do this straight away it may be difficult to point to this incident as making it impossible for me to continue to work my notice - how many days have been worked since?
Anyway, having made up my mind (acting reasonably) why I have to leave straight away then I would put the reasons in the letter - but they have to amount to what any reasonable person in my situation would consider a "fundamental breach". When I turn up for work I would say "I'd like you to read this letter now please", and hand her the letter. I would probably do this without going in to her house, and if she told me to come in I would say "I'd rather not, please read the letter which will explain why'.
I'd probably have my DH (well, DW in my case) standing at the end of the drive. If after reading the letter she says she wants to talk about it say yes, but I would like my DH to come with me.
If this happened, my DH must not join in the discussion. His only role is to be there so that I can turn to him and say "I do not feel that I am being treated fairly/I feel that I am being bullied/whatever and I would like to leave now.", and then he can make sure that I leave without further discussion.
Then I'd go and get some professional advice straight away (because none of the matters in the letter are clear cut, but they should alert the MB to the fact that I know that I have rights and intend to defend them) - if I couldn't get through to ACAS or a CAB, I'd find a lawyer that operates the free half hour consultation scheme.
I would definately not take a tape recorder, this suggests that you are trying to trap someone or catch them out, and that their behaviour is not totally unacceptable because you are prepared to accept it as long as you have a tape recorder in your pocket. Might be useful if you were looking to build up a case of persistent low-level bullying over time, but we don't have time for that here.