access /wp-admin/post.php

I see there have been previous posts about getting a 403 page with the text ‘You don’t have permission to access /wp-admin/post.php on this server.’ when trying to save or publish a post.
The problem did not seem to be resolved previously. I had the same problem and loading my page incrementally until the problem arose showed that it was the word IFRAME in the post text that was causing the problem.
Posting this so it might help someone.
Mark