Maximum allowed repayment calculator?

knightstyle

Thanks all, yes I understand all this. I was just wondering if there is a calculator to tell me what I can oay each month to achieve this max. overpayment each year?  The monthly payment chart I have shows a slight increase in the actual amount that goes towards repaying the mortgage as the interest goes down so if I am allowed say £3k max extra repayment 3000/12 is an extra £250 a month to add to the payments. 
 But in year 2 I will have to reduce the monthly overpayment. So will have to change the DD every year.