SOLUTION FOUND!! Courtesy of Whoathemes
So so simple!! I fixed this issue, across several different sites, by simply dragging and dropping the gallery box below the Yoast SEO box on the page edit view.
I don't know why, but having active galleries and where the gallery box was above the SEO box caused SEO data not to be saved against the page.
Phew! I can now continue to enjoy the benefits of Yoast with confidence. Was getting a bit frustrated with not being able to get anything back form Yoast though. buts its free, so hey, cant complain too much!! :)
Just glad i found a solution, hopefully it helps someone else.
James