Formula correction

Hi guys,

This should be a fairly straight forward one for you. Hopefully!

I've been making a Statement of Account, but I've kinda broke my formula. As far as I can see it *should* be working fine, but just isn't! Can anyone shed some light on it for me please?

=IF(ISERROR(INDEX('[Sales 2013-2014 Kym Edit.xlsx]April 2013'!$B$3:$K$50,SMALL(IF('[Sales 2013-2014 Kym Edit.xlsx]April 2013'!$C$3:$C$50=$A$8,ROW('[Sales 2013-2014 Kym Edit.xlsx]April 2013'!$C$3:$C$50)),ROW(2:2)),5)),"",INDEX('[Sales 2013-2014 Kym Edit.xlsx]April 2013'!$B$3:$K$50,SMALL(IF('[Sales 2013-2014 Kym Edit.xlsx]April 2013'!$C$3:$C$50=$A$8,ROW('[Sales 2013-2014 Kym Edit.xlsx]April 2013'!$C$3:$C$50)),ROW(2:2)),5))

It's supposed to list all invoices for a given client, but it's being a pain where I thought it would be a relatively straight forward task.

Secondly, the Sales 2013-2014 workbook from which the data is harvested has a worksheet for each month and my formula is only good for one. At this rate I'll need to have 12 versions of the above; is there any way I can tweak it so it will aggregate all 12 months data?

Any advice is hugely appreciated.

Here are the files in question:

Statement of Account - dummy data.xlsx
Sales 2013-2014 - Kym Edit.xlsx

You can attach the file in this website itself, refer my signature area to know how to attach a file

Because not everyone will have access to outside links