potterton profile 50e pilot light?

I have a potterton profile 50e. my heating system wont come on, when i switch it on the pilot light doesnt come on. how can i get my pilot light to come on?usually when you switch it on you can hear the tick tick noise of the pilot sparking up, but when i switch it on you can only hear the noise of the system starting up, but with no pilot light, the heating doesnt come on.