This gets odder as I investigate further, and I have been a bit more methodical.
If I take a blank drawing, and draw two graphics, e.g. an ellipse that is partially overlaid with a rectangle, then select both and do a Crop, is seems to work quite well. The ellipse is cropped as expected, and I get the cropped bit overlaid with a light pink colour rectangle - the rectangle disappears when I deselect the image, and the cropped image remains. Selecting the cropped ellipse shows the pink rectangle again and both can be moved around together OK. This is understandable behaviour and I can live with it, although I would like to get rid of the rectangle altogether and just have the cropped bit left to move around and duplicate.
Doing the same thing on my drawing behaves differently - I have about 20 graphics (nothing special), but am doing the crop on an empty portion of the drawing. I crop as before, but now get a darker pink rectangle with an even darker red rectangle inside bounding the original graphic (the ellipse). If I grab the ellipse (i.e. the dark red bit) and move it, it moves but the cropping rectangle stays still, which results in a different part of the ellipse showing when I do the deselect ( or none at all if I moved the ellipse completely away from the cropping rectangle - in this case the ellipse can't be individually selected and cannot be found again unless I Ungroup). In order to move the cropped image unchanged, I have to grab the cropping rectangle and move it, at which point both ellipse and rectangle move together and the cropped image doesn't change.
Doing a Duplicate on the cropped combination gives more strange results, and I am going to want to do this as I proceed with this drawing. But I'll leave that for now.
Paul.