Well I have yet to find any flask that keeps your tea or coffee hot all day. Over the years I have had many different kinds, and with everyone of them the drink is just about warm come lunch time.
I have found that by taking the milk along and adding it at the bank side the drink stays hotter that little longer. I have a couple of different flasks at he moment, but i am not impressed with any of them, and the food flasks are even worse.
Kelly kettle, or the stove for me if i am going all day, nothing worse than a cold tes or coffee.