Expanding a range across multiple sheets

Hola!

I'm in need of a little pointer/advice.

The following formula extracts invoice data for use as part of a 'statement of account' workbook, with separate sheets for each month.

=IF(ISERROR(INDEX('[Sales 2013-2014.xlsx]April 2013'!$B$3:$K$50,SMALL(IF('[Sales 2013-2014.xlsx]April 2013'!$C$1:$C$50=$A$8,ROW('[Sales 2013-2014.xlsx]April 2013'!$C$1:$C$50)),ROW(2:2)),5)),"",INDEX('[Sales 2013-2014.xlsx]April 2013'!$B$1:$K$50,SMALL(IF('[Sales 2013-2014.xlsx]April 2013'!$C$1:$C$50=$A$8,ROW('[Sales 2013-2014.xlsx]April 2013'!$C$1:$C$50)),ROW(2:2)),5))

It's working beautifully for 1 month (April 2013), but I would like to modify the above so that it uses the same range for all 12; 'April 2013', 'May 2013', 'June 2013', 'July 2013' etc etc.

Any feedback gratefully received.