Counting Unique Values based on several criteria

Hi

First post here.

I'm working on a spreadsheet (Excel 2010) and I need to count the number of unique values in a column (K), where other values on the same row meet a set of criteria. I've managed to successfully SUM all the values in another column when they meet the criteria, but counting unique values is proving beyond me

Here is my formula so far

=COUNTIFS(LabourData!D:D,'Summary '!A$5,LabourData!O:O,H5,LabourData!J:J,CONCATENATE(">=",$B$2),LabourData!J:J,CONCATENATE("<=",$F$2),LabourData!G:G,"LAB")

What I think this says is Count if;

Column D = A$5 and
Column O = H5 and
The date in Column J is between the two dates specified in B2 and F2 and
Column G = LAB

How do I add the criteria that Column K should be unique?

Thanks

Adam