Ok, when I use the field generator to create a hidden for either post_url, or post_title, what happeens is, it works, but it creates a duplicate, on is named the hidden-XXX (XXX the number that gets assigned) and the other is named post_url and post_title respectively.
Weird, but it works.
So, if I cane the hidden-XXX name to one I want, it stops duplicating, but them stops working altogether (the value is just post_url or post_title)
I have 3 hidden above them using the add_filter method to drop in dynamic variables which all work slenderly.
But this, is driving me nuts.