You're under no obligation to pay it back if you don't want to. And why would you want to?
Haven’t so far.. I wrote them an email that I should be paid out at least for my notice period (1 week during probation).. was told as I agreed to leave immediately (well as it was untenable on my part and soul destroying to stay and didn’t make sense) I’m not owed my notice. Not sure of my legal rights.