Borate, it would appear that I have 'cracked' this problem on my own. I could not come to terms with suggestions of cropping [I have no wish to remove any parts of my content] or zooming [which effectively does the same thing, one only uses part of the content]
No, after selecting a series of content 'frames' I invoked 'Image Effects' and selected 'letterbox to aspect ratio' in the 'Aspect ratio Transforms' section and then simply left the aspect ratio setting as 'Auto Aspect Ratio' and voila everything 'got left alone'. My problem solved. It would be nice if there was a 'button' to 'select all content', but I shall manage quite well with the present select with 'Ctrl' pressed, or selecting and 'shift' selecting at the end.
I should add that I have the 'main' aspect ratio set to 'Match Content' which effectively sets the aspect ratio to that of the 'largest' or perhaps 'common' aspect ratio in the project. And also, up to now all involvement has been with stills, I have yet to tackle the [occaisional] movie clip I shall be adding to the sequence later. One step at a time.
I thank you again.