My Worcester Greenstar 24Ri stopped working?

My Worcester Greenstar 24Ri stopped working and it wont fire up. The Burner green light comes on and the system sounds like it is starting up but after 25/30 second later it goes off again with some knocking/rattling noise coming from somewhere at the top of the boiler. I have taken the front cover off to look at the air pressure gauge but I cannot find it. The manual seems to suggest it might be a flue problem - either the air inlet is blocked or the inner flue is leaking gases into the air inlet.
Does anybody have any idea of what the cause might be and can you help? Is it possible that there is an airlock in the system and if so how do you get rid of it.