Assets image paths

Just updated a site from Perch 2.7.7 to 2.7.9 - like the changes to Assets but when I add an asset I now get a broken image.

I am using buckets.php for two buckets - Images in /public_html/assets/img/content and pdfs in /public_html/assets/pdf. Looking with ftp I can see the image has been loaded into the correct bucket but its not had a file-thumb@2x.jpg file created. The path in the Assets screen is showing as /assets/file-thumb@2x.jpg - I only have that set in config define('PERCH_RESPATH', '/assets');

Appears buckets.php might not be working. Any ideas?