My Worcester Greenstar 24Ri won't fire up. The Burner green light comes on as if ready to fire up but it goes off after a 25/30 seconds. At this point there is some knocking/rattling of the pipes and soon after everything stops.
Does anyone recognise the problem and can you help?
If it is an airlock in the system how does one get rid of it?

GNBDG43, February 2014