The DVLA seem to be making a habit of this at the moment. That is, they return driving licences to those falling under the HRO scheme without first insisting on a medical and then request the medical many months later. I am afraid the medical is compulsory and whilst it might feel unfair, the reality is that you have "got away" with 15 months of driving without satisfying the requirements of the HRO scheme when you should have done so.