Try something like this

Change range and month to your range and month.

Col A has Dates
Col B has your account No e.g 001
Col C has value before Vat (Change range to Inc Vat range)

=SUMPRODUCT(--($A$19:$A$27>=--"01/07/2006"),--($A$19:$A$27<=--"31/07/2006")*(--(B19:B27="001")*(--(C19:C27))))

You can learn more about Sumproduct on the attached link

http://www.xldynamic.com/source/xld.SUMPRODUCT.html

VBA Noob