After following a very similar guide (here) I was able to get the desired results I was after.
Hope this helps the next person.
The expressions I used in the below screenshot are:
Compose Function
Inputs : formatDateTime(triggerBody()?['dueDateTime'],'MM-dd')
Note : I'm using just the dueDateTime value here.
In Planner, I'm just using the quick card which only has the 'Set due date' option (along with task name & assignee)
Create Event (V4)
Start Time : addToTime(outputs('Compose'),8,'Hour') - This sets the event start time to 8:00am
End Time : addToTime(outputs('Compose'),9,'Hour') - This sets the event end time to 09:00am
