Ah, I didn't really explain that well. The three half hour periods are selected based on the system wide demand. You are then charged for your demand during those periods. So you pay a lot for using capacity when the network is strained. Also, for distribution charging (rather than transmission) there is a half hourly congestion charging mechanism. Every HH period gets put into a red, yellow, or green band and you pay more distribution charge during the red and yellow periods.