Hi Ragnar
I'm glad you're going to have someone to help you with your tribunal.
I can't tell you whether you should tick all the boxes, because this is outside the scope of the forum and I have no idea what is in the letter. Staff who work in Law Centre's are generally well respected. I think you may have to take your CAB's advice on this one.
If your doctor will provide you with a supporting letter for your Tribunal then you should get one. It's good that he is willing to do it.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems