Thank you for the replies and comment. I have tried again and have now been successful. I think the problems have been due to whether or not a new Layer was present. I have files of differing formats open e.g .TIf and Paint.net.
I am not experienced in using Layers and I think there is a need for rather more Help on these sorts of topics for occasional users please. For instance I didnt understand for quite while that in order to have two or more images open on the screen at once that you had to open in a new window. My current problem is how to make two different plans exactly the same