Even the greatest attempt to lump a bechamel sauce seems to be reversed by a good whisk. This evenings attempt at an all in one method with flour not cornflour was salvaged. I added all the ingredients ignited the hob, was distracted by something I needed to google and retuned to boiling milk but a harsh whisk makes a sauce that I am sure will suffice for a kids macaroni cheese.
Elizabeth recommends making a roux with the butter and flour adding warmed milk cooking then cooking again in a bain marie.
She is yet to convince me that my method of a lump of butter, a bit of cornflour and a slosh of milk is any worse off.