This regards HTML code inserted using the plugin. It is showing as invalid because it contains "&" instead of "&". If I open up the code insert editor and replace it with "&" then when I save it it reverts to "&".
I managed to make the HTML valid for now by putting "&" and saving it. But making any change in the editor and saving again causes it to revert to '&'.
It seems to be automatically decoding HTML entities. I'm tempted to edit the plugin so that it stops doing this, but please could you advise if there is a less hacky way to deal with this situation?