Hi Skylark. I do have some experience of this in that in my 50s I was in a similar position to you, highly stressed and suffering with acute anxiety and, like you was signed off with stress. I too was pushed to go back too soon, and because of this ended up signed off again for a longer period of time, as I was on the point of a breakdown. I ended up having to change my job and it was only after I had left my previous position that I realised how much it had contributed to my stress and anxiety.
I can only say from my own perspective, but I would consider the advice from the union rep. I understand it doesn’t sit comfortably with you, but having been there I’m sure you’re aware you won’t be thought any more of if you push yourself to go back and make yourself worse. Having said that I understand your concern about the new job, but as you have a spotless previous health record and have explained your present job is causing you stress maybe you could approach them and explain the situation? A really difficult situation for you but your health is the most important thing.