Problem igniting my water boiler: Vaillant Direct Power Mag?

Hello, can anyone help?
I have a gas water boiler Vaillant Direct Power Mag , mounted in the kitchen which has started to give me problems.
Most times when I turn the hot water on in the bathroom and occasionaly even in the kitchen, the hot water does not turn on.
I can hear the sound of the motor trying to start it up automatically(as it should with this model) but then the flame does not ignite and there is no hot water.

Other times , expecially in the kitchen, the hot water comes on.

Can you tell me if it will be very expensive and difficult to fix? What can the problem be?

thank you very much

Anna
February 2010