more than one outstanding invoice with the same company

Hi

I have more than one outstanding invoice with the same company. I have received some of the monies owed, but these were not allocated to a specific invoice. If i allocate them to the oldest invoice a balance would remain due on that invoice together with the full amount on the other invoices. all my invoices state "payment due by return", as no payment terms were agreed. My questions are:-

Do i raise a late payment letter for each invoice and charge the compensation fee each time.
On the invoice which is part paid, how do i set out the interest calculation.
With my statement on the invoices of "payment due by return" how many days after the invoice date should i leave before calculating the interest.

Thank you

Thank you David

with the part payments being received, do i have to calculate the interest on the reduced balance left after each part payment or do i continue claiming on the full amount until the debt is paid.

Regards

clive