bit late a reply as you say your assessment is today, hope it goes well, but I think you can send in supporting evidence after the assessment itself as a decision isn't supposedly made until the assessors report is sent and then all evidence is before the decision maker to make a award.
My Parkinsons local advisor wrote a lengthy technical detailed letter on how Parkinson's effects me personally
My PD nurse also wrote a detailed supporting letter on her interactions and observations of myself
Both were happy to do so.
Hospital letters (most recent) also included
I also wrote a personal letter on how a typical day effects me personally living with parkinsons which is noted by the decision maker as helpful.
There was no cost involved with supporting letters though i understand some gp practices do charge a fee for medical evidence , I usually get a copy letter sent too me of appointments with Neuro & Nurse.
make sure you make copy's, you can ask for a report of your PiP assessment.