Thanks for the clue to working this out.
so I can find it later.
solution to 'YJ Image Slider for K2 can not delete items'
There was a 'Forbidden error' in Chrome Developer tools console.
for the delete.php file
The site has Akeeba Admin Tools installed and needed an exception to allow direct access to this file.
more details here
This is the file
this enables deletion of YJ K2 Images in front end and backend.