Potterton Puma 100/100e over pressurized water overflow?

I have a Potterton 100 / 100e. The boiler water overflow is constantly flowing water, and the system water pressure was slightly in excess of 3 bar. I turned the water inlet off, and the power off to the boiler to reset it. I then turned the power and water back on, and the pressure is approx 2.5 bar, though the overflow is still flowing water indicating that it is over pressurized. The pressurize overflow is now always running if the water inlet is on. How can I stop the overflow?