Why will my Ferroli Tempra 30 gas boiler not shut down?

My boiler is constantly burning, whether the timeclock is instructing it to switch off or to switch on. It also continues to burn even though the thermometer temperature is exceeded, so the house is like an oven. The only way to switch it off is to isolate the electricity, so in the morning the house is freezing. I have even removed the timeclock, but it continued to burn, so I can assume that it isn't a faulty time clock. Can anyone help?

Gareth, Belfast
February 2010