Help with formula for accumulative payments due by calendar month

Hi I have attached the work sheet I am working on what I would like to be able to do is use a formula that automatically adds the new months mooring fees to the total owing (D) from the first of the month.
I would also like to know is there a way I can make more than one payment in a cell say for example Customer test 2 pays £250 for Jan moorings on 2/01/15 then pays another £250 on the 10/01/15 how could I split the cell horizontally so that all the calculations still work? Can I divide the cells some how between F2 and H2 to allow for mutiple payments?

Really appreciate any help!

The easiest way I could think of doing this would be to track payments separately, so you would have a sheet for payments and another to track the overall/monthly status of the accounts. I needed to add a helper cell at the top (highlighted yellow) with the months' number equivalent but you could hide that row in the finished product. Also, with this solution you would have to be careful of entering the customer name so it matches exactly, I was having trouble with the formula at first because customer 1 had a space after the 1 which was throwing off the sumproduct function (this could be negated somewhat with more in-depth formulas using trim). If this isn't what you were looking for, let me know and I can try to readjust.

Thank you for your help