I have now found this, which may be useful for others here!:
What if I have already paid for a full year's TV Licence but am due to turn 75 before it expires?
Don't worry. We can convert your licence to a Short Term TV Licence, which will cover you until you qualify for a free Over 75 licence. If you have made any overpayments, we will send you a refund once your application is complete and your free Over 75 TV Licence has been issued.
To convert your licence to a Short Term TV Licence, please call 0300 790 6165 with the following information to hand:
Your name and address.
Your date of birth.
Your National Insurance number.
Your current TV Licence number.
Alternatively you can complete the online renewal form and we will send you a refund when we issue your free over 75 licence.
Except of course, that I obviously don't get the refund until NEXT YEAR!