Thanks Lynn,

Date part was used as it is documented as determenistic.

previously I had used:

DATEADD(dd, (DATEPART(dw, Date_Received) - 1) * - 1, Date_Received)

Unfortunately the formulae provided are not determenistic either and the first is only correct when @@DATEFIRST = 7 Although this is not ISO.

I think the problem there in lies and that using DW is what is causing the issue. Am going to do a manual calc of weeks. between 19000101

and @date to work out which work day it is when startdate = monday. This hopefully will be deemed determenistic.

Regards