For killing fungus in lenses and on old slides I use carbon tetrachloride (p a u s e for Health and Safety Police to jump in!) . Usually the fungus returns on lenses sooner or later, but of course I never know whether that's the original fungus that wasn't totally killed or whether it's a new outbreak. Carbon tet. used to be used in dry cleaning, so I wonder whether it's replacement would help if you sent the pouch for dry cleaning? The cleaners should be able to advise on any potential ill effects on the leather (I have no idea). Of course, the main thing in coping with any fungal problems, whether relating to lenses, leather or, indeed, Athlete's Foot, is to keep the environment as dry as possible! Not easy in the UK.
Good luck!