Anyone with any good tips. have seen one on youtube using hydrogen peroxide but that's my last resort

Leave it in a sink full of water and some bleach. Works wonders on getting tea stains off the teaspoons, should work on your flask. Don't forget to give it a good rinse in cold water after it been in soak for an hour or so.

Soda Crystals .. Brand new!

Excellent job done with the bleach-- didnt have any soda crystals to try. Cheers. should keep tea warmer for longer now

Bleach isn't good on steel long term, I use sterilising powder used for home brewing, something like this:

http://www.brewbitz.com/VWP-Cleaner--Steriliser---400g__p-1067.aspx?gclid=CMeL6qHDmMMCFWoOwwod4lgAaQ