I have received the following correspondence: 

This service was re-tendered by Strathclyde Partnership for Transport earlier this year and potential contractors were offered two options, one at the then-current hourly frequency requiring three vehicles and one at a reduced frequency needing two buses. We tendered for both options, the two-vehicle one naturally being significantly cheaper than the three-vehicle timetable, and SPT ultimately selected us as the successful bidder and the 90-minute headway as the timetable. This came into effect in July for a period of three years.
 
I have subsequently discussed the issue raised by your constituent with SPT and voiced my support for re-instating the hourly frequency if funding were to become available. However, the final decision rests with them.