Overpayment

Nonnadiluca

I have a five year fixed rate mortgage and can overpay 10% of the year's starting balance each year without penalty. Last week I made a payment of 10%. Today I've had a letter saying that a part redemption has reduced the mortgage balance and therefore the monthly payment, and to contact them if I would like this to be applied as an overpayment instead as this will reduce the capital balance on which interest is charged, but will not reduce the monthly payment.
I called but couldn't talk to anyone, was told that I will get a call back within 48 hours.
What I would like to know is, if the monthly payments stay the same, I will be paying off over 10% of the starting balance so will I be penalised? Hope that makes sense and someone can advise.
Thank you