Potterton Housewarmer 55. How can I fix it?

Roaring noise and random ignition?
My boiler like many makes the roaring noise on igntion. I have replaced the burner and the gas valve but it still does it although it is now intermittent. Since I have replaced these parts the boiler now also ignites randomly for about a minute, even though the timer is set in the off position. Apart from this the boiler although about twelve years old is in excellent condition. Any ideas anybody?