[google sheets] - SUMPRODUCT within ARRAYFORMULA

[manc]

Dear Demigods,

Hope that I can post this here.....

I'm using Google Sheets and I have the following formula in column O which is copied down as far as the eye can see:
=ARRAYFORMULA(SUMPRODUCT((GOODSOUT!$A$2:GOODSOUT!$A$1004=GOODSIN!C2)*(GOODSOUT!$L$2:GOODSOUT!$L$1004)))

Google Sheets has a neat way where you can put the arrayformula in your column header and it replicates the formula to all rows. This is what i want to achieve with:
={"COLUMN HEADER";ARRAYFORMULA(SUMPRODUCT((GOODSOUT!$A$2:GOODSOUT!$A$1004=GOODSIN!C2)*(GOODSOUT!$L$2:GOODSOUT!$L$1004)))"}

Not quite working though.

Any ideas?

Best regards
manc