Kaz, it's on the young side for transitioning to one day sleep, but that could be it. Some mums find that it can help to make the morning sleep a little later to see what happens. Another idea is to try alternating - one sleep one day and two sleep the next. Sometime babies seem to need this before they are ready for one sleep all the time.

I hope you work it out soon. Tired, cranky babies are really hard work.