Formula needed to sum multiple match criteria

I am needing a formula to sum info from 1 book to another based on two lookup criteria. Its the sort of info you can easily get with pivot tables or filters but my colleague needs a copy and pastable formula so it can be automated.

There are two workbooks Order Index example.xlsxCash Flow book example.xlsx; one called "Order Index" has a list of orders placed along with supplier name (Col C), date due for payment (Col H) and value of order (Col I). The other workbook, used by a colleague is called "Cash Flow" and contains a list of supplier names (Col B) with row 1 containing months (1st Jan 2014, 1st Feb 2014 etc).

Under each month on the "Cash Flow", I need to show the sum of orders due for payment within that month, for each supplier named (in Col B). In other words, sum the figures from the "Order Index" based on supplier name and date due for payment.

Often, on the Order Index there is more than one order per month from a supplier and the dates due are specific days (10th Jan, 21st Jan etc) and these need summing up for the 1st Jan 2014 Column in the Cash Flow book.

Can you help?