Victoria
I'm surprised your GP has asked for such a document, it runs the risk of being described as your leading them in any letter that they might write, I would include a statement in anything that you send to them saying that they should only include information in their letter that they in the medical judgement and knowledge of your problems that they agree with.
As to what to send them, basically, you want them to comment on the problems you have with meeting the Descriptors in each of the ESA activities, so look at each activity and describe your problems in terms that match the relevant Descriptors. So as an example;
Due to my Arthritis in my ankles knees and hips, I am unable to walk more than 50m due to significant pain in my joints. Also, due to Arthritis in my hands and wrists, I am unable to self-propel a manual wheelchair over any distance due to severe pain.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
