He Andy,

You’re welcome!

My solution depends on the autosave feature that seems to be called each time you press the preview button. However it seems that sometimes saving the preview takes more time than loading the preview so you might end up with either an older preview or no preview (the current version). It usually does provide better results compared to using get_the_content() since that sometimes returns a preview made by another user that previously edited the page, or no preview at all (the current version again). Also, post_parent is better omitted, I saw instances (in the database) where post_parent was either 0 or the current post_ID so that is a bit buggy / random.

I hope you find a use for this fix, however I haven’t seen anyone else having a problem with previews since the support topic from december 2013 so your solution seems to working fine for most users.