Found the problem - the output of link is being cached in the web browser.
We should be able to eliminate this by a modification to the file
plugins/fields/eldownload/gwejson_downloadfile.php
by adding
header("Cache-Control: no-store, no-cache, must-revalidate, max-age=0");
header("Cache-Control: post-check=0, pre-check=0", false);
header("Pragma: no-cache");
just before the
$byteswritten = readfile($file);
at the end of the file