A day walk is one of the quietest pleasures of British life, and the picnic you carry with you is half of it. Not the grand affair of hampers and folding chairs, but something honest and small that fits in a rucksack: a flask of tea, a hunk of cheese, a few oatcakes, an apple that will taste better on a gate than it ever did in the kitchen. The trick is to pack with the same care you would give to a good book — nothing extraneous, nothing missing, everything in its place.

Start with the flask, and let everything else follow

A flask is the spine of a British picnic. Fill it with tea so hot it would scald you at home, because by the time you reach the top of the hill it will be exactly right. Fill it first thing, before breakfast, and let it stand while you get your boots on. A one-litre flask does two generous mugs, which is the right amount for two people on a walk of any length.

If tea is not your habit, a flask of coffee works just as well, and in high summer a bottle of water chilled overnight in the fridge will still be cool four hours later if you wrap it in a tea towel. Water matters more than anything else in the bag. Carry more than you think you need, particularly on the downs and the moors where the streams are not for drinking.

• One flask of tea or coffee, filled to the brim and sealed tight
• At least a litre of water per person, more if the day is warm
• A small bottle of squash or cold tea for anyone who finds plain water dull

The food: simple, sturdy, and better than it sounds

The best walking food survives being jostled. Oatcakes are ideal, being firm enough to carry a slab of cheese without cracking, and they have the plain, oaty flavour that suits a hillside. Cheddar, cut thick, is the classic partner. If you want something softer, a small tub of something spreadable travels well if you wedge it upright between the flask and the fruit.

Fruit should be the sort that comes with its own packaging: apples, pears, a couple of satsumas in winter, a banana early in the day before it bruises. A handful of dried apricots or a small bag of nuts will revive flagging legs on the last mile. If you want a little sweetness, a few squares of dark chocolate or a couple of flapjacks will do, though on a warm day chocolate is best kept in the coolest part of the bag.

• Oatcakes or good bread, cut thick and wrapped in paper
• A slab of cheese, cut at home rather than on the hill
• Two pieces of fruit per person
• A small handful of nuts, dried fruit, or both
• Something sweet, wrapped separately so it does not scent the rest

The things you will be glad you remembered

A groundsheet is the item most often left behind, and the one you miss most. A rectangle of lightweight plastic, or a piece of old oilcloth, weighs almost nothing and turns a damp tussock into a lunch spot. A folded bin liner does the same job in a pinch and doubles as a waterproof for the rucksack if the weather turns.

Carry a cloth to wipe your hands and a small knife for cutting cheese. A couple of paper bags, one for rubbish and one for anything that comes back dirty, will stop the inside of your bag from becoming a mess. Take a spare bag for every crumb — it sounds fussy, but a walker who leaves no trace is a walker who is welcome back.

• A groundsheet, or a bin liner in a pinch
• A tea towel or cloth, and a small folding knife
• Two paper bags: one for rubbish, one for anything mucky
• A couple of napkins, or a folded square of kitchen roll

Packing the bag so nothing suffers

Weight low and close to your back, soft things in the middle, hard things against your spine only if they are flat. Put the flask upright in the middle of the bag, wedged by the food so it cannot roll. Fruit goes on top, where it will not be crushed. Anything that might leak, including the salad dressing you were sensible enough to leave at home, should not be in the bag at all.

A simple order works well: groundsheet at the back or strapped beneath, flask in the centre, food around it, water in the side pocket where you can reach it without taking the bag off, and the rucksack liner or waterproof cover on top. If you are walking in the rain, pack as though the bag will be set down in a puddle, because it will be.

Eating well, whatever the weather

In winter, eat early while the tea is still hot, and choose a spot out of the wind, even if it is not the prettiest. In summer, eat late and high, once the sun has lost its bite, and find shade under a hawthorn rather than a wall. Spring and autumn are the walker's seasons, when the light is long and the going is good, and a picnic at noon on a dry tussock is about as fine as life gets.

Wrap up warm before you sit down. It is surprising how quickly you cool once you stop moving, and a spare fleece or a sit mat will keep the lunch from turning into a shiver. Eat slowly, drink the last of the tea, and let the view do its work.

A last look before you latch the lid

Run through the list once before you close the rucksack: flask, water, food, groundsheet, cloth, knife, bags, something warm. Then add a book, because a day walk is a bookish sort of pleasure and there is no better place to read three pages than a stile in the sun. Latch the lid, tighten the straps, and set off. The walk is the point, but the picnic is the reward, and both are better for being kept simple.