Upon switching the hot water and central heating switch to the on position, our back boiler kicks in and seems to be building up pressure for a couple of seconds before a flame licks out of the top of the boiler (behind the fire) and the boiler starts heating up the water. Ive never noticed this flame before and it has me slightly worried. It is due for its annual service. There has always been a build up 'noise' when we click it on and every so often whilst its on for a while with the 'whummppp' noise (Burner i assume...) that seems to start the heating up of water. It is during this 'whummppp' noise that i see the flame lick out the top! Any ideas? Cheers in advance. Matt