Evidence needed for tribunal

Well this is the second time I am having to go to tribunal for failing my medical. The first time I had what I thought was lots of evidence about my conditions and problems that can arise with them but was told at the tribunal that the evidence I sent was not what they wanted. So now I am in a situation where I have to go to tribunal again and I have absolutely no idea what evidence would help me.

I tried to get a letter from my doctor and was charged just under £16 for two lines stating that I had diabetes type 1 and two episodes of coma in October 2008. No other medical history was added and can I just add diabetes definately isn't the only illness I have.

I'm wondering if anyone knows what kind of evidence I should be trying to get? My medical is at the end of April. I have also rang Citizens Advice and the woman there said I needed medical evidence from someone in the medical profession saying whether or not diabetes, overactive thyroid and tachycardia could stop me walking 800 metres, sitting in a chair for 2 hours or walking up 15 stairs without resting the thing is I don't even think a doctor could write a letter like that as everyone is different. I have a friend who has diabetes type one (not the other medical conditions) who works full time, cycles every weekend and goes to the gym 4 nights a week.

Any help would be so very much appriciated. Feel free to ask me any questions.