SSRS 2008 Report - Hide Textbox based on a value in another DataSet

Hi there. I am trying to hide Textboxes and Tablix's in my report based on values from other DataSets. For example on the first page of the report, 4 Tablix's (DataSets 1-4) are printed. On subsequent pages, Textboxes and other Tablix's (DataSets 5 onwards) are printed but I want to hide/show these based on a field from DataSets 1-4. I sort of have it working but my expression is wrong because some items are still being printed, I think because it has put FIRST in my expression:

What is the correct expression to use?

Thanks

BM