It occurred to me later after posting the above, that at one point i used calibre to save epub to epub, to sort a different issue. I wonder if thats what affected my margins. I'll add a screenie of the current (if I can), of a book (not from the store) which used to have margins wider than the text.
You'll see in the attachment, the first is an approximation of how it used to be... and the second is how it is now. I have no more complaints about margins, whatever the reason for the fix.