but when I turn the boiler off at the switch and turn it back on it comes on again??
Does this happen every time you bleed the rads? If so possible you are taking off more than air and the water level is dropping and the automatic filler isn't filling it fast enough. All this is highly unlikely though it could just be a coincidence.
The system depends on back pressure to operate safely. If the pressure is released, as it would be when you were purging the lines, that indicates a potential health and safety hazard. The system shuts off to avoid problems. You should find the manual that came with the system and read it to figure out how to do that task safely.
The boiler is detecting a drop in pressure and is shutting down to prevent damage
You have a sealed system boiler that requires a minimum pressure to work properly and safely. When you bleed the rads(something you should not need to keep doing)the pressure drops and switches off the boiler,when you turn off and on again you are resetting its electronics. You will eventually likely damage the boiler if you keep doing this.If there is a problem with it get it looked at properly as its cheaper than replacement boiler.
