You are the man !
I had to tinker with it slightly, but this is the working code:
for /f "delims=" %%i in ('type 2009_07_01.csv ^|find /c /v ""') do set Count=%%i
set /a Lines=%Count%-1
for /f "tokens=*" %%a in ('type 2009_08_01.csv') do (
if !Count2! EQU 0 (echo %%a)
if !Count2! GTR %Lines% (echo %%a)
set /a Count2+=1
) >> 2009_08.csv
The only thing I need to try and work out now, is how to set the dates automatically.
i.e. On the first of each month, this batch file uses the current months data against the previous months.
Or, to do the same using the latest two files regardless of the names. (Extension would always be .CSV)