Run the ‘DEMextract.exe’ tool with the following options:
-i “inputfilename.txt”		The name of your input file
-o “4thGreenElevations.txt”	The name of the output file
-c 1000				Number of columns in input file (default is 500, but we need 1000)
-pos 595 540			X, Y co-ordinates calculated above as the starting point
-s 50				Size of the square to be snipped

This will take about 1-2 minutes, depending on the speed of your processor. It is written in VB.NET code, so is fairly slow! 

There is only limited error handling in the tool, so if the parameters are wrong, the programme will exit with a generic error message.
