If you have your card, which is what you're placing the X in the box to say you have, then you can check the expiry date. It's like EHIC cards, that also last 5 years.
However, while there are campaigns to remind people to renew their EHIC cards, until these past few weeks, I've never seen anything to rimind people to renew their Medical Exemption Certificates.
And while people with type 1 diabetes continue to need insulin etc for the rest of their lives [unless they get a pancreas transplant], several people with type 2 diabetes on these boards have been able to give up medications for diabetes, which means they lose their right to free prescriptions.