dont know why but i had a play with the code and it now works
Dim cust As Variant
Dim bvalue As Variant
Dim stotal As Variant
cust = Nz(DLookup("[Factory_Stock]", "tblTool", "[Tool_ID] =" & Forms![frmBookOut]!Tool_ID), 0)
bvalue = Nz(DSum("[Book_Value]", "tblToolBooking", "[Tool_ID] =" &...