On some images you can press the yellow button to dispaly the "Start Time Selection" menu. That chooses the stream that is viewed. The screen will be blank on all NVOD channels until you choose a stream
The "yellow button function" isnt available on all images; and seems to be missing from a few of the latest ones... the alternative way to do this is to brose to the channel using the "webinterface" - that way you can choose the programme to watch.