The installation is not complete - the boiler has yet to be properly installed until the radiators get warm.
Each radiator has two valves - inlet and outlet - turn them all off on all but one radiator - it should get warm. If it doesn't get warm then no water is flowing around the system and the boiler will protect itself from an explosion by making the overheat thermostat trip out.
If water is being pumped it goes via a valve (external to the boiler) to either the hot water cylinder or to the radiators and sometimes shared between the two when the valve is in the Mid Position.
It may be that your diverter valve is stuck in the hot water position - find the valve and move the control by hand - see what happens.
As for a blockage whoever told you that is pulling your leg (or incompetent) - there was no blockage before the new boiler was fitted and the radiators have been flushed - a blockage would cause the overheat safety thermostat to trip.
I suspect a defective diverter valve - I am not a boiler engineer and have no expertise - like many boiler engineers i just guess...
Good Luck..., January 2017
