Multiple (3) SUMPRODUCT conditions

Hi

I'm trying to SUMPRODUCT multiple criteria but am obviously doing something wrong.

It needs to be a SUMPRODUCT as the source data will be stored elsewhere and will not be open at the same time as the COSTS sheet. I don't need a pivot table either, for other reasons.

Essentially what I need is...

In cell E7 of the costs sheet, put the total Spend from column O on the source sheet...

IF,

The Project in Source column B = COSTS E1 - and
The Category in Source column P = COSTS B7 - and
The Month in Source column Q = COSTS E4

So, the total Associates spend for Project1 in January

I'm currently using this formula but getting a zero (not N/A or VALUE)
=SUMPRODUCT((source!B:B=COSTS!E1)*(source!P:P=COSTS!B7)*(source!Q:Q=COSTS!E4),(source!O:O))

Your help would be hugely appreciated as always.

Many thanks in advance

Tim