Our header tank split and emptied overnight which I didn't discover until the following morning when the heating failed,the boiler lights showed that it had overheated and switched off.I installed a new header tank but the boiler now operates for 50 seconds and turns off with a knocking sound that suggests to me air in the system.Bled all radiators to no avail.( Christmas ruined)
depending on which version you have there is either an auto air vent or a manual one which is on the return pipe right hand side. but make sure your pumps working
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.