I recently had a letter, phoned up ready to kick off. The issue was resolved very quickly, apparently the prescription admin service has changed hands/ has been centralized and is being managed by another team; as such they are chasing up accounts which may need updating/ are out of date. In my case I hadn't updated them with my current address. Mine is due to expire this year, apparently I need to advise my GP to apply for exemption on my behalf.
I only really remember having the exemption cert in the last 5 years, I think prior to that, the pharmacist would see diabetic items on a script and it would go through no questions asked. No doubt somewhere along the line people have abused this "trust" hence the stricter management and certification.
Its important to remember we (diabetics) are entitled to free prescriptions, and hopefully they are not questioning this.