noise in pipes as if overheating?

we usually have the heating set at 40 deg but last night the radiators got a lot hotter than normal and we had knocking noises from the pipes, my guess is that the water is overheating. i've turned the thermostat down on the boiler but it doesn't appear to make any difference. if the boiler did reach too high a temperature wouldn't it shut off? is it worth buying a thermostat at approx £15 before bringing in an expensive plumber?