The /f/ sound is spelled with <ff> or <gh> as in rough. The /ŋ/ sound is spelled <ng> or <ngue> in tongue.
The contrast is between a voiceless labio-dental fricative and a voiced velar nasal continuant, and only ocurs in medial or final positions. It is not a problem.
The mean density value is 0.3%. The list makes 21 semantic distinctions, a loading of 70%.
bailiff bailing bailiff baling buff bung buffing bunging buffs bungs buffed bunged Cardiff carding cliff cling cliffs clings fluff flung gaff gang gaffs gangs gaffer ganger gaffers gangers huff hung luff lung luffs lungs Prof prong riff ring rough rung slough slung sough sung stiff sting stiffer stinger Taffy tangy tiff ting tiffs tings tough tongue whiff wing whiffs wings
John Higgins, Shaftesbury, October 2010.