Count unique values with specific criteria

Hi
I am working in a 2007 spreadsheet and am looking to count the number of unique values in column A, dependant on a date range in column B and against each organisation in column C.

To make it a bit clearer, you will see from my attached workbook, the "Stats" sheet has the list of each organisation and I want to enter the formula here which will show the number of sessions (unique count of column A in "Data" sheet) within each quarter Q1 01/04/12-30/06/12 and Q2 01/07/12-30/09/12 (date range in column B in "Data" sheet).

I'm sure this must be able to be done but I just don't seem to be able to work out how, will probably kickk myself when I find out!!

Any assistance is much appreciated.

Many thanks

Karen