I'm an exams officer and have seen so much misinformation flying round about children missing exams so thought I'd start a thread with some advice!
1- don't panic it's unlikely to be the end of the world.
2- contact attendance (or whoever you report absences to at your school) and then email the exams officer. Don't insist on talking to them (and don't yell at the receptionist if she says they are starting an exam so can't come to the phone). If you want a call back ask for one and then chase if you don't hear from them after a couple of days.
3- don't buy a GP letter!!! You don't need one. If you have anything like a GP appointment confimation, GP appointment transcript, prescription you can take a photo of,, medication covering the illness that you can take a photo of then email that to the exams officer as evidence.
4- if your dc doesn't need to see anyone (if they have a vomiting bug etc) then ask for a JCQ self certification form, fill that in and send it back to the exams officer.
JCQ Special consideration doesn't have to be done until several days after the end of exams so there is no rush to get it all sorted on the day.
If your dc is doing a non JCQ exam then the process might be different so ask for advice from the exams officer. Just remember it's a really busy time of year for us and we might not be at our desks to answer calls or emails everyday so don't be afraid to chase but be patient.
Most of all reassure your dc that it's not the end the world, illness happens and can't be helped and the exam board will help as much as they can. Sometimes looking after your health is more important than exams and that's OK!