Milton — Thermos flask

I am a Principal in a private school.
I had purchased a Milton vertex flask of 750 ml, blue colour with blue glass, about 6 months back and used it for a week. Now when I filled it with hot water, the flask became completely hot outside and the water-cooled down after 3 hours or so.
I do not have the warranty card so I have no way to prove that it is still in the warranty period.
I therefore request you to lodge my grievance and replace the flask at the earliest.
I shall be grateful for your kind gesture.
Thank you.
Simmi Oshan Saikia.
[protected]@yahoo.com