Appointment time value limit in drop down

Hi,
I've set the time base to 15 mins in the 'General' tab.
I'm trying to select a time value for a couple of our 'Services' to 165 mins and 180 mins respectively and when I click the dropdown field in the 'Services' Admin page I appear to be limited to 15 - 150 mins.

Is there a way to extend this?

I've read the FAQ about the time base but I don't want to add a 2nd time base I just want to extend the time options for the 15 min slots.

e.g. currently on the 'Services' page in the Admin panel I can select 15, 30, 45, 60, 75, 90, 105, 120, 135, 150 and I would like to extend so I can select: 165, 180, 195, 210, 225, 240. for a 'Service'.

Is this possible?

Regards
SteveB