Thanks for your response, that was an approach I was taking. The issue being you first need to create the field [Last Working Day] in order to reference it.
So, looking at some simple code it won't work (unless I'm miss interpreting your answer?)
Load * Inline[
Date ,Working Day Flag
,[Working Day Flag]
,if([Working Day Flag] =1 ,Date,previous([Last Working Day])) AS [Last Working Day]
Drop Table Data;