which part needs replacing when my sime 80E boiler does not relight?

When the heating is on and i go to turn on the hot water tap my Sime combination 80E boiler does not heat the water, i.e. the flame goes out, or it heats the water and when i close the tap off the heating does not come back on, i can hear a noise from the boiler but there is no flame heating the rads!! I have to switch the boiler off and then put it back on again and will light up and heat the rads! What is the problem????