Thanks John, I.Ronnoco & last but not least, Divya's website on date calculations in sql for your help in pointing me in the right direction ... for anyone with the same issue, the ssis expression did not require a condition, also the dateparts had to be all the same to return the previous days date no matter if the day had transitioned into another month.

Corrected SSIS expression is shown below:

"X:\\WNTS\\Reporting\\List28old "+ RIGHT( "0"+ (DT_WSTR, 2) MONTH( DATEADD("dd", -1, GETDATE()) ) , 2 ) + "-"+ RIGHT( "0"+ (DT_WSTR, 2) DAY( DATEADD("dd", -1, GETDATE()) ) , 2 ) + "-"+ (DT_WSTR, 4) YEAR(DATEADD("dd",-1, GETDATE()) )+".csv"

Finally thanks to Darren Green's article on SSIS date expressions at sqlis.com for his comparison of T-SQL Date functions and their SSIS equivalents.