Totaling A Subform

Hi there hope you can help :)

I have created a form with a subform and has the following information displayed:

(Form)
Tenant ID Mobile Telephone Number
Title Credit Reference
Surname
First Name

(Subform)
Date Due Amount Paid On


(Form) Total Of Arrears -

I need text box at the bottom of the form to caculate each tenants arrears.
To show if the tennant has any arrears there will be no date shown in the [Paid On] box (If there is a date in this box it means the tenant has paid and therefore no arrears) so the [Amount] for that tenant owes will be totaled up in the box at the bottom of the page [Total of Arrears]. Also a tenant might miss a payment on one week and a pay another rent payment on time. Please could you show how to do this in access and not by VB code.

I've attached my database - well thanks for looking and hope you can help

HeadintheClouds