Delays to new trains on Barking to Gospel Oak line - and to 15 minute service?

Rail magazine issue 612 (page 9) says:

"Delivery of 12 two-car class 172 Turbostars to London Overground will be late ... The diesel multiple units were due to be delivered in the summer according to Transport for London spokesman Guy Pitt, but the delay means the Class 150/1s currently used on the Gospel Oak-Barking line will remain in place until the new units arrive. Pitt says this will not ... impact on the planned 15-minute timetable on the railway due to be introduced later this year. 'We are not quite moving into contingency territory yet,' he said."