Thanks WLLady. I did not check the whites of her eyes (too late for that) but WWW I did dig her up and the kidneys were almost the same yellowish colour as the liver, not sure if they would change colour after being dead for a while- and perhaps a bit bigger than I thought chicken kidneys should be. WLLady I didn't see anything that looked like hemmorhages on the outside, but I didn't cut or look that closely.
In retrospect I did botch the autopsy, I was really expecting gallons of fluid to gush out and was really surprised when there was none- I didn't have a good plan B.
Next time hopefully there isn't one, but I have her chicks so you never know. I'll try and do a better job. I can just picture myself with the chicken belly up on the morgue slab, the skull cut open with a saw and the brain in the scale
Sorry just a bit of black humour, actually I am very sad, I am very sorry

to lose this hen. I felt badly that I had killed her for just being fat, but now I do feel a little bit better thinking she really did have serious issues.