22 mm Gas pipe?

Hi, Re Ideal H E 30 Boiler,(2 1/2Yr Old) should 22mm gas pipe be fitted right up the boiler, mine reduces to 15mm about three feet away.
The reason i am asking is because my boiler, every so often, wont start up and displays L F (Flame Failure) when i press the reset button it lights the boiler with a bang.
I would be gratefull for any comments.
Thanks Bill.