Thanks to the group for all your help. I have two approaches that work and some tutorial about layers. As to my immediate problem, I can press on. But I want to dig deeper into this and understand why the two approaches work. My problem is not actually done - I can read the files OK. But after I have modified the content, I need to write it back in the same format. But I think I am OK for now. Again, thanks to the group. Stu23