Sumproduct across several sheets

Hi All,

I'm looking for some help in obtaining a cumulative total of quantities across several sheets. I've attached a test workbook for reference.

I had used the vlookup + vlookup but seem to be returning errors on too many occasions

Sheet 1 contains the summary that details the 'Item' (items will vary across sheets depending on the customer) and quantity, I am needing this to check the other sheets and perform a sum of the matching item

Many thanks
Stuart