It sounds to me like your best bet is to ask all that when you see the CAB as you're going anyway.
But hopefully someone with experience can come on here and maybe give you their experience
I forget anybody (sorry) are you in touch with your local benefits adviser? Mine is indirect contact with my psychiatrist, so he arranges the supporting evidence for me (a letter) and it's free through him.
If you're in Kent PM me and I'll tell you who mine is. He's based at the social services office so maybe you can phone your local office and ask to speak to the benefits adviser? Or if you are in touch with your local mental health team you could ask them.
Failing that, like I said, the CAB should know.
All the best to you