HI MATE.
IF YOU TAKE THE FRONT OFF. TURN POWER OFF ON THE WALL.
PUT THE CENTRAL HEATING/HOT WATER SWITCH TO THE MIDDLE.
TURN THE TEMPERATURE DIAL TO THE MAXIMUM.
IF YOU LOOK AT YOUR BOILER YOU WILL SEE AT THE BASE STICKING UP ON THE FRONT A BUTTON (ON MINE IT'S BROWN).
TURN THE POWER...