how to fix my vokera 24e?

the red light, ignition light keeps coming on and all we get is cold water. i have to reset it quite often, british gas have been out twice once to have a look and then they sonic cleaned the heat exchanger, however it just keeps happening, they recommend a full system power flush which will cost over £500. I reckon that it may be something wrong with the ignition system side of things. Any help would be great if.