Thanks Ramesh -
I worked it out must have had a typo earlier in my replace. For those of you who are interested here's my code
REPLACE ("D:\\DataMart_Reports\\Today\\" + @[User::AppName] + (DT_WSTR, (30)) (DT_DBDATE) DATEADD("DD", -1 ,GETDATE() )+ ".CSV" , "-","")