Thanks or the pickup, sorry I was retyping the formula and changed sheet names due to a spelling mistake. This is not the problem however as the formula is actually
=SUMPRODUCT(--('Weigthted Delta Array'!A:A=A2),--('Weigthted Delta Array'!B:B=B2),--('Weigthted Delta Array'!C:C=C2),('Weigthted Delta Array'!D:D))

please excuse the spelling.