Interest calculation on rental arrears

I am the landlord of a commercial property with a tenant who is in significant rental arrears. As per their contract interest can be applied to the value owed.

Their tenancy started in 2015 and to the present day, cumulative arrears are still the issue. It must be noted that interest cannot be charged on interest owed on this bill.

As can be seen from the attached payment schedule, I am seeking to have the correct interest applied to their account with the payment being made going against the initial outset of money owed

Should you have any further request for more information prior to starting the project, message me directly.

I will prefer this to be completed on microsoft excel or in a format which I can continue to update once you have completed your works