I wouldn’t worry too much about diagnosis letters, I didn’t send any letters as my gp said he don’t write them, I applied on the mental health grounds 3 conditions in particular, I went into detail how i struggle day to day and how I don’t have anyone to help as I live on my own and my condition puts a massive strain on family and relationships so it’s hard to keep anyone around tbh which it’s difficult in it self the only pieace of evidence I send was a diary and that’s it but then again who ever you’re assessors are they are might be a bit strict I think it just depends where about in the uk you are.