Hi
I do this regularly and have been told by various AMEs and Gatwick that the times all run from the date of examination.
If otherwise, the CAA would be allowing you to fly for longer simply because you held a class 1 - the class 1 does not mean you are more healthy, nor is the examination more likely to pick up pathology in the common areas such as the cardiovascular system.
Logically your examination for a Class 1 also gives you a Class 2 which runs for the same length of time from examination as if the examination had been for a Class 2
I will now wait to be shot down for my logic!