I'm on my second playthrough going for everything. So far on my marks i've got 5 star rating for all of them (done quite a few). I have just reahed the tower and going through the compulsory marks but got a 4 star rating for one like an idiot, saved over my normal save game. Can I come back here and do the mark again to improve the rating?

Yes. I think that once you defeat the boss in Oerba you can go back and re-do any marks.