my mains 24e baxi potteron make will not light up?

i have mains 24e gas boiler 2 weeks ago i found that hot was not as hot as normally it gives .i called the engneer and he told me that heat exchanger was blocked up that it needs to be replced ,so he instaled new heat exchanger. the hot water went back to normel hot .but the boiler would not fire up every time .flame failur light started to come up .so by reseting the boiler few times it would fire up and and some time work all day no problem it would fire up every time then other time it did not matter how many time i reset it just would not fire up then doing nothing diffrent it fires up and works no problem,i called the engneer and explaind and told me to flush the central heating system .which i did but it has made no difrence.then he said give couple of weeks for the heat exchanger to settle in .but that has not done any good.
my boiler is 5 years old he said these are cheap boilers they only last 5 to 6 years .now i have boiler which is running on it own will i have no control .its try to run as i can hear the pump running but just will not fire up .it show flame failur .can any one advise me on this please
thanks in advance