It does sound like a lot of water! DD sometimes drains a 260ml bottle during the course of the night, but during the day she doesn't drink as much as I would like. N seems to be having a lot! Definitely get her checked out for diabetes so you can cross it off assuming everything's OK.
on the TT front Vic! Great work. From what I have read most bubs are still in nappies overnight until nearer to 3 so I wouldn't worry about that.