at union rep!
This isn't about whether you've 'got a case' or not. This is about you feeling harassed and it not being addressed and dealt with in a prompt and appropriate manner. I am not impressed that your union rep is not interested in helping you unless you are bringing some kind of legal case.
This is all obviously causing you great distress, which makes me as I do think if this had been taken seriously, dealt with quickly and appropriately straightaway, you and everyone else could have moved on effectively. As is it, it's dragging on, you are not feeling listened to, and I think as a result this is escalating far beyond what was originally necessary.
Is your new manager keeping in touch with you properly while you are signed off? My inclination would be to see if he can meet with you - if I were advising him from an HR point of view I would advise him to visit you at home, so hopefully this is possible. Have a chat with him, say you are frustrated by the feeling that you are not being taken seriously, HR are being really slow, not acknowledging your correspondence, and dragging it all out, you feel unnecessarily.
Say that you are disappointed that you are not able to work at the moment, and frustrated because if this had all been addressed quickly and appropriately, you feel you may well have been able to put it all behind you, move on and contribute to the new team as you would like to do. Tell him about the most recent correspondence you have had with HR and ask whether he could speak to whoever the most senior HR person is to find out what the delay is and push for a quick resolution.
He is your line manager and if there is an issue which is affecting you like this and preventing you from working, it is his business and his responsibility to you and to the organisation to do what he can to get it resolved. Obviously you need to take responsibility as well, and do what you can, which you have done. But if you are not getting a satisfactory response, and are having trouble with HR, your line manager should be the one to push them and find out what the problem is. It's in his interests as well as yours anyway.
See if you can have a chat with him, face to face if at all possible, make sure you emphasise your desire to get back to work but your frustration that the lack of response to your problem is delaying your return.
And with regard to returning to work, if you are not feeling up to it to return on 5th November, just make sure you see your doctor, he/she should sign you off for a bit longer if you need it.