Found this one on some expats websites with relatively good reviews, so I went straight to the ER (old building not the new ones as it’s easy to get lost) and I was not disappointed: friendly staff, good doctor, they spoke English and when they couldn’t we used translator apps.
A couple of hours later, I had an IV and some meds and was sent back to the hotel.
So yeah if you’re unlucky enough to find yourself not feeling well in Yogyakarta this is a good place to go.