Thanks for the Idea, finally got it working with:
=VLOOKUP(SUM(IF($A3<=Sheet2!$C$2:$C$73,IF($A3>=Sheet2!$B$2:$B$73,Sheet2!$A$2:$A$73)))-1,Sheet2!$A$2:$C$73,2,0)
Sheet2 data, like you say was just a lookup table, of teh possible periods and a unique number. Working fine now thanks :)
Scott