Okay, so I've pruned the sql somewhat to this, the idea being to remove all the blank cells before inserting them into the main table, Tbl_Costing:
INSERT INTO Tbl_Costing ( [Job Code], EmpName, HoursWorked, WeekEnding )
SELECT Tbl_Trial.[Job Code], Tbl_Trial.EmpName, Tbl_Trial.HoursWorked...