What to do re: interest

Hi all,

My question is about a customer / individual debt. As in working for a member of public, not a business / commercial debt.

I’ve read on a few places that when interest is added to an overdue amount and the customer makes a part payment, interest and any fees are paid off first, then the overdue amount.

Eg. Overdue amount £1000 + interest £100 = £1100
The customer pays £1000, so this pays off £100 of interest and fees and £900 of the overdue amount. This leaves £100 of the overdue amount and would continue to accrue interest.

Is this completely right and if so, would anyone know where I can find details of the law / regulation etc that stipulates this?

Thanks