Foosty up here, and yeah, Milton fluid, or I'm told that sterident tablets work very well indeed too
cheers,
Toddy
On the few occasions I use a flask I only ever put in boiling water (unless it's soup in the food flask). Tea and coffee are terrible for leaving a taint on flasks. Also with boiling water you can use it for tea, coffee, noodles, stockcube drinks - whatever takes your fancy at the time. But as said if you can soak the stopper in milton it should help.