I have now solved this
This was done by using the following expression
RangeSum(Before(Count([Days to post return]),0,ColumnNo()))
Days to post return was built into the load script as a DateDiff between Week Sent and Week Return this is then used in the expression.
the load script is like this
[Days to post return] = DATEDIFF(Week,[Week Sent Date],[Week Return Date]) /1