This question comes up in a variety of ways and the threads often go off topic with irrelevant calculations. The interest charged each year is exactly the same - we will return to this. Reducing the repayment is simply reducing the payment you are contractually obliged to make under your...
answering just the question asked, throw it against the mortgage as a lump sum. Leave your term the same; if you can keep the payment the same as well, you'll clear your mortgage even faster.