mears, as you know,I think I work in the same Trust as you. We recently had to have a first "disciplinary" meeting with one of our Junior staff whose performance had been less than ideal in certain aspects. The HR manager at our end insisted that the junior member of staff be given the opportunity to be accompanied, either by a legal representative (though it wasn't that serious a problem) or by a friend or colleague. You absolutely must have someone with you, if that is what you want, and, if your manager feels threatened by you, she should as well!
In our meeting the HR manager acted as "referee", and seemed to be following a set pattern, IYSWIM. We gave our point of view, without interruption, then he gave his response, similarly without interruption, then we were able to answer some points, then he was. HR manager proposed a plan of action, and when it was agreed by both parties the meeting was over. The plan will be put in writing and sent to both sides, then a further meeting will be scheduled to see if both sides are happy.Please don't get me wrong, I'm not suggesting that you have done something to be disciplined over (if anything, it seems as if your manager has!), but I would imagine that that is the sort of pattern that any Grievance procedure would follow.
I don't know if this process applies to you, exactly, but I suspect that something similar will happen in your case. Would it be less threatening for your manager if you were accompanied by a friend rather than a Union rep? Or has it gone too far for that sort of compromise?
BTW, I haven't a clue who you are!(Although I've probably given enough clues as to who I am over the last few months- ), but if I can help in any way, please let me know, on or off board.