power light fault?

I have a intermittent fault with my halstead ace high which has been going on for the best part of a year. The heating will fire up, radiators will get hot then the heating will cut out and the green power light will just keep flashing and then the radiators cool down. I have had many engineers over the months but they are just guessing when it comes to finding the fault any ideas would be great as i am fed up of calling british gas.