So i have done some further testing.
Waiting and trying reboots seems to make no difference in my environment.
The 64G card showed 19G for the overlay above but after a re-image was only showing 130M.
I tried the method outlined here Out of Disk Space (overlayfs seems incorrect) - #18 by akelge but this caused the card to be non-bootable.
A 16G card I had was not resizing either so i used the resize2fs method and that worked fine.