Hey! Thanks very much for your replies. From reading all three responses I worked out that the program was taking data files from a folder named data - so I moved what was in there and put in a new data file. After a few attempts trying out your ideas, I got the program to work and produce an output file that looked sensible to me. I never thought I would be able to sort this out - your help made all the difference. I am very happy - thanks again!