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Eventually I found out what the problem is and it is caused by the child theme that I had created. Another problem elsewhere that had an extra blank line at the start of a news feed and Demerit's comment above suggested this as a solution. When I changed to the base theme all was fine. Going back to functions.php for the child theme I found a blank line outside  (though not at the start of the file which is why Demerit's suggestion didn't lead me to the solution before).