Urgent help

Hi i would be very grateful if anyone can help me out. I started getting chest pleuretic pain 3 months ago and it got worse after 3 months and i was tested positive for TB. Im unable to get a refund for my exams because the deadline for refunds is gone and i want to take the 4 technician exams in december. Im on 6 month TB treatment now. AAT told me i can miss my exams and write a letter after to explain that ive been ill. They said i need a sick note or i think medical certificate. Do i have to write a letter and get it signed from my GP or do ijust get a medical certificate from my GP. Please help!

All help appreciated.