[AlexBell]

Thanks to you both. I'll try your suggestions

Well, it's fixed, but I really don't know why or how.

The only things I changed were to make a new png image from the jpg provided by dwig, and to remove the setting to ignore margins - and yes, I know I should have done them separately to get a better idea of what was happening.

My best guess is that telling calibre to ignore the wide margins may have affected this the first image in the book - none of the other images which had exactly the same markup were affected.

It may be that as dwig said the original png was the problem. But I am as sure as I can be that same image worked without problem as I developed and tested the conversion.

Anyway it's fixed. Thanks again for all your help.