Definitely take two 1 litre flasks (rather than one 2 litre as they cool so quickly) but only ever filled with boiling water!
Then its Assam/Yorkshire tea bags with freah milk added....Also take coffee occasionally..
As a change, and many wouldn't try this, I take a tub of Marigold Swiss Vegetable 'Bouillon' powder which is just instant vegetable stock which makes a delicious hot drink!! You don't need much and its a very, very satisfying......
I buy the Organic variety and its available in most supermarkets. Give it a try!
In the old days it was tinned Tomato Soup which was like a food in itself-just great!!
ps You cannot drink tea after having the stock drink as an oily film settles on the tea-not nice!
