My Sabre 24 boiler is erratic!?

i am a tennant just moved in and the bl**** boiler ignites sometimes and other times it just purs. what's up with it? the green light comes on and it purs and nothing happens, OR it automatically switches to a red light for no reason. The tennant last got charged because the boiler man said it was the nature of the boiler! i am angry that the tennat should get charged for something that is clearly sh*t*. anyone help? Thanks Matilda