Requesting supporting/evidence letter from Dr.

I am applying for pip and I requested my rheumatologist (hospital consultant Dr.) to write me medical evidence/support letter. They told me to write them an e-mail explaining my request (what do you want?). So do you have any idea what I should include in my letter .Please advise
Thanks

Thanks Gary for the information.
Is that a good idea to request supporting letter from the consultant. Some people suggest i don't need to ask a letter from my consultant. The DWP or capita will do it themselves if they want.. Please your advice
Thanks

Jonathan,

I’d suggest obtaining any supporting letters you can and not only include them with your form, but refer to them in any relevant questions. Every piece of information you provide works in your favour. Don’t assume the DWP will make any efforts to corroborate your case.