I'm trying to improve the following line of code so that I can
substitute different file names when necessary, but can't get the
syntax right with the Indirect function.

=SUMIF('[1 Feb 06 - 30 Apr 06.xls]Sheet1'!$E:$E,B21,'[1 Feb 06 - 30 Apr
06.xls]Sheet1'!$AA:$AA)

The spreadsheet that this line is in runs to roughly 100 lines, and a
similar formula is used in two other cell locations on each row. The
spreadsheet that it references (1 Feb 06 - 30 Apr 06.xls in this
instance) runs to roughly 13,500 lines.

If I use a different spreadsheet (e.g. 1 May 06 - 31 Jul 06.xls), then
it means changing the above line roughly 300 times for 600 occurences.

What I have been trying to do insert '1 May 06 - 31 Jul 06.xls' into
one cell and then use the indirect function within the sumif. In this
way I would only have to make one single change to the entire
spreadsheet.

Any help would be appreciated.