I have exactly the same problem on my ideal classic ff260 boiler , A gas fitter looked at it a week ago and took the red pipe off the top of the fan casing and blew down it - you could hear the air pressure switch operate . He then put the pipe back on and the outer case .... and it now lit ok , . It has been fine for a week , but now it has began to do the same .
leighton
January 2009