how can l fix my white knight tumble dryer?

when l turn my tumble dryer on l can hear it clicking as though it is going to ignite and l can see a flame but then the flame goes out, or something puts the flame out, the drum still goes round but there is no heat, l have to stand their opening and shutting the door untill it eventually ignites but then when the thermostate gets to the right temperature and clicks off the dryer will not re-ignite again. We have fitted 2 new solenoid gas valves. Any help would be much appreciated. Many thanks