Talk about a slap in the face. You win the war - send Hitler and Hirohito and the rest of them packing - and what happens? The government hits you with bread rationing. Everyone knows in these days of excess that the British were never fitter than during their finest hour. But rationing did not end with the cessation of hostilities. Far from it. No sooner had peace broken out than they halted supplies of American dried egg and banned “luxury” fruit. When poor world harvests in 1946 meant bread was rationed for the first time, some murmured that life might have been better under the Nazis.

Incredibly, it would be 1954 before rationing finally ended, with fats and meat among the last items to be liberated. With “austerity” the order of the day, life in post-war Britain was no picnic.

When things did pick up - when the brakes eased off and the sun came out and people were able to venture once more on to the beaches and into the fields that they had defended with blood, toil, tears and sweat - what goodies were waiting to be spread on the tartan travelling rug? Why, the same Enid Blyton fare that had been popular all along. Sandwiches, hewn out of loaves from a local bakery, contained a variety of fillings besides the usual cheese and ham, ranging from corned beef and tongue to the meat and fish pastes (tomato and anchovy anyone?) sold in glass jars by the Chichester firm Shippams.

These were joined by a popular wartime American import that had put down roots and decided to stay: luncheon meat. In its most famous form - canned luncheon meat - the spiced ham, or Spam, that George Hormel of Hormel Foods had invented in 1937 was credited by the Russian leader Nikita Khrushchev as having enabled his army to defeat Hitler. Years later, its ubiquity, satirised in the Monty Python “Spam, Spam, Spam” sketch, would result in the name doubling as a term for repetitive, unwanted email.

But in the late Fifties and early Sixties, email was a long way off. For this was still a world of homemade sausage rolls and a welcome flask of tea: a world as yet unfamiliar with the tang of pure fruit juice (it was lemonade or squash for the kids), a world largely without plastic, where everything was wrapped in greaseproof paper and transported in canvas or wicker shopping baskets. Even crisps, whose salt came in little blue twists, were sold in paper bags, and not by Walkers or Golden Wonder, but by a Lincolnshire firm called Smith’s.

But if the Famous Five had stuck around a little longer, they would eventually have noticed some disturbingly modern elements creeping into the familiar picnic scene. To begin with, these were mostly about technology, as materials and processes that had been developed before the war began to make themselves useful on the home front for the first time. Aluminium smelters, built to supply the aircraft industry, were now able to feed the demand for foil, and this, together with polythene, which had been accidentally discovered by ICI in 1933, was to transform food packaging and retailing. In 1960, airtight, leak-proof Tupperware food containers arrived in the UK, transforming the look and feel of the British picnic almost overnight. And a year later saw the invention of the Chorleywood process, a method of aerating bread on an industrial scale. Within a few years, thousands of independent bakers had been bought up by a handful of giant flour milling concerns: big boys whose stock in trade was the sliced white loaf.

Industrial processing and the dominance of large companies was beginning to change other aspects of the picnic experience, too. Triangles of Kraft Dairylea processed cheese, which first appeared in the early 1950s, were joined in 1960 by Philadelphia cream cheese, also produced by Kraft. Bright orange processed cheese slices were easy to slot between pre-cut squares of white Wonderloaf. For added convenience, there was Heinz sandwich spread, and, for afters, Lyons Kup Kakes (orange, lemon and original chocolate), individual fruit pies (the square ones) and Penguins (the first biscuits to be sold by name).

All good comfort food for a population that had never had it so good? Maybe. But by the 1960s, there was an appetite for change - an appetite that would not be satisfied by the occasional slice of quiche lorraine and a scoop of avocado pear, the new-fangled fruit that would inspire a generation of drab bathroom suites. In the event, it was the boom in foreign travel, together with the arrival from Asia and the Far East of thousands of displaced families with a yen for catering, that broadened the British taste. But while the holiday love affair with European diets and the rise of the Chinese and Indian restaurant at home would drastically alter British dining-out habits, the most dramatic changes in al fresco eating would take place in an altogether different context.

Close your eyes for a moment, and imagine yourself in a large field. You are sitting on the grass, surrounded by other people. Somewhere, there is music playing. But never mind that for now. You must eat. So what will it be? Something hot, something cool? Something spicy or something that sizzles? Open your eyes and follow your nose. Grab whatever you fancy, and bring it back here. For this is a kind of picnic, the sort of outdoor meal that people have been enjoying at rock concerts and folk festivals and all manner of mystical events and alternative happenings right across the British countryside for the best part of half a century.

Eye-witness accounts of the appropriately named Battle of the Beanfield in 1985, in which police violently dispersed New Age travellers preparing to observe the summer solstice at Stonehenge, invariably mention that a number of food stalls had been set up on the periphery of the proceedings. What began in the 1960s with fish and chips and the occasional burger has now developed into the sort of culinary scene that makes the posher kind of Ascot picnic look like something from an Enid Blyton story.