Ok great. I see what you're saying with regards to the control source etc and that I need to refer to the fields directly. But how would I perform this calculation as the orderoverheads and postagepaid only apply once for each invoice?
If I use this in the report footer then it will Sum those 2 fields for every order line within every invoice which is fine for the first two fields but not orderoverheads and postage paid:-
=Sum([transuk]-[productoverhead])-[orderoverheads]-[postagepaid]
Many thanks