stainless thermos

UN-scented White King bleach. Don't make it to strong, about 50:1, but you will have to do it more than once. Possibly even three times
Bleach actually works best at between 15°C and 25°C, so water straight out the tap at this time of year should be ideal.

Thanks very much exploroz,Bat.

Just throw enough dish washer "liquid" in the thermos to coat the inside - leave it for a day and your 'tea" stains will fall out when you rinse
kes

Use some CLR, cleans my thermos, & coffee purculator, pour a little bit in with some water, give it a good shake then rinse it out really well, & doesn't leave any after taste, regards, Dave.

Fill thermos with boiling water, add one teaspoon of Sterodent powder(used for cleaning false teeth) leave for about one hour then rinse. Will shine like a new one!! Used this method for many years.