Hiya, is time keeping part of her contract or maybe an unwritten commitment that you all have in regards to your preschool? and do you feel that the night work could or has affected her competence to work during preschool hours?
Frustrating for everyone if you feel she's not viewing your group professionally.
Working hours from Acas, would giving them a call to explain the situation & to see what they'd advise help?
Best wishes, let us know how it goes xx