I just got my licence back today after passing cat C, but I’ve just noticed my expiry date has had 2 extra months added to it, I’m 40 years old at the moment and I sat the medical in November 2014, when I got my provisional C the expiry date was 16/12/2019 which was the day before my 45th birthday as I expected it to be, but the expiry date now on my full licence is 06/02/2020 which is 2 months after my 45th birthday.
I knew if you hadn’t had a licence issued since 2012 you would get 5 years on it before you needed another medical even if it took you to over 45, but it seems you get extra with any change as I’ve now had 2 licence changes since 2012