Hi Maz,
Yes, have you, by chance, put the calendar on the person in question on their employee profile but not on the task that's supposed to adhere to the 30 hour week?
The reason I'm asking is the Gantt doesn't read individual calendars set on employees but rather reads the global calendar set on the Gantt View Pro settings and the calendar set for individual tasks on 'Calendar' in the advanced tab. The latter is where you'd select the 30 hour calendar for the employee, which should then translate to a correct depiction on the Resource Utilization. See example in attached screenshots.
Please let me know if this solves the problem.