...Works with years greater than 1899, though it's quite easy to change it to make it work with older years too:
SELECT myISOweekyear, myISOweek,
7*(myISOweek-1) + DATEADD(yy,myISOweekyear-DATEPART(yy,0),0)+ 3 - DATEDIFF(dd,0,DATEADD(yy,myISOweekyear-DATEPART(yy,0),0)+3)%7 AS Monday_datetime...