Pip

Just tell them how your illnesses affects you each day. 
Goodluck

Remember the answers you wrote in your application form as these will be needed to be justified at your acessment, do not be bullied into doing any movements that cause you pain or discomfort, caution, the accessors know the distances from nearest bus stops, car parks train stations if in walking distance. So when asked how you arrived say by car or bus the acessor will know how far you have walked. Even in the center it’s they have CCTV watching in and outside the centres and the distance from waiting room to acessment room. Good luck and request your acessment report a week after your acessment, this will give you a guide as how your award may be. 

@philipcarson you dont have to if you dont feel fit. The assessment medical isnt designed to cause pain, its merely about how your condition affects your ability to carry out simple physical movements. I was asked to lift both arms above my head when i attended mine, i only did the left because i explained that just lifting my right up to my face set off an attack and my assessor was fine with it. My universal credit work capability assessment the assessor asked me to do something similar and he quickly added in "but i dont expect you to use your right". hope that helps x 

best of luck! let us all know the outcome x