Heritage used to be easy. It was stately homes. It was cathedrals. It was tea towels in the gift shop and buttered crumpets in a National Trust café. It was nostalgia. Not any more. Over the past 50 years, during which British society and our towns and cities have been utterly transformed, ordinary people have fought to save the streets, buildings and landscapes that mean so much to them. In doing so they have completely revolutionised what we mean by heritage.

Heritage has democratised. These days, it can mean pretty much anything: a coalmine, the childhood homes of the Beatles (now owned by the National Trust), that little café down the road with an interior straight out of Expresso Bongo. It can even be a building which to many is pig-ugly. Take Robin Hood Gardens. For the past year a battle has been raging in East London over plans by Tower Hamlets to demolish and redevelop this 1960s Brutalist housing estate. Passions run high. Architects and preservationists are pitted against council and developer. Yet if, 40 years ago, you’d have said that this slab of concrete was heritage you’d have been laughed out of the planning department.

To understand how heritage went from chocolate box to concrete box, I’ve been filming a seven-part TV series for BBC Two, Saving Britain’s Past. It was the experience of the Second World War that created our basic understanding of heritage. Before the Blitz there were, astonishingly, no proper systems or records for preserving our buildings and landscapes. There had never been any need, because the British landscape, at least the oldest, most cherished parts of it, had changed so slowly.

Admittedly the Industrial Revolution had so transformed much of the country that the glimmers of a conservation movement emerged through campaigners such as William Morris and his Society for the Protection of Ancient Buildings, aghast at the modern world’s impact on the old. But compared with what was about to happen, the industrial revolution was small fry.

That all changed when Hitler embarked on not only the Blitz, but also the infamous “Baedeker raids”, a bombing campaign targeting not military or industrial sites, but those of cultural value listed in Baedeker’s guide books. Cities such as York, Exeter, Canterbury and Bath were bombed just because they were beautiful. Looking at the archive footage of Bath’s destruction in April 1942 is a grim task. Besides the human suffering, 19,000 of the city’s buildings were wrecked, including such gems as the Royal Crescent and the Circus.

The attack sent the country into panic, triggering a sense of collective ownership of our landscapes, the same drive that brought into being the welfare state and the NHS. John Betjeman proposed a national buildings record, the Ministry of Works began a salvage scheme of historic buildings that needed urgent repair, and the 1944 Town and Country Planning Act gave birth to the lists — Britain’s first inventory of buildings of national or historic importance, graded I, II and III according to their significance, to be protected.

Heritage was born. The Ministry of Works appointed 30 architectural historians to compile the lists. These were traditional, nostalgic, conservative. Things not quite up to scratch included architecture from most of the previous century, certainly all things vulgarly industrial. But at least it meant that what Britain looked like in the future would no longer be left to chance or be so vulnerable to attack.

What is remarkable is not simply the country’s speedy acceptance of the idea of saving heritage, but how enthusiastically we have done so. We are an intensely nostalgic country, especially in our post-imperial decline. Yet conservation is not always conservative. It can be downright radical.

Ever since it was invented, this cosy idea of heritage has been whittled away by those it excluded. Just as our understanding of history has diversified from kings, queens and great men to the social history of ordinary people, so what we choose to feel passionate about has shifted from cathedrals and castles to the 1950s cafés in which our quiffed teenaged mums and dads tried to be cool; to the coalmines some slaved in, and the council estates many lived in. My heritage wasn’t a 14th-century village church or a Georgian mansion but a postwar school built by the Hertfordshire schools building programme, a strikingly modern place jam-packed with welfare-state optimism. Can’t that be preserved alongside the 14th-century village church? Why can’t the everyday landscapes most of us live in be heritage?

These ideas began to arise in the mid-1960s just when British society was loosening up and admitting grammar-school politicians, gay playwrights and working-class pop starlets to its higher echelons. They even came up in Bath. As the council started tearing down Grade III listed Georgian streets — the only grade then not legally protected — not even the nascent heritage bodies noticed. But ordinary people did. In the mid-1960s Peter Coard began drawing the little human quirks of the artisans’ cottages and shopfronts disappearing around him and co-founded the Bath Buildings Record. It took another decade for cultural grandees such as Kenneth Clark to catch up with this battle by the little people. Coard unearthed a brutal fact: there was a class system in heritage.

In London John Betjeman became the first secretary of the Victorian Society in 1958, but it took another decade for the Victorian to be thought of as anything but vulgarly industrial. In early 1970s London an alliance as radical as CND or the antiVietnam rallies, of Marxist activists, gentrifiers, old market porters and West End actors inflicted the first big defeat against the planning establishment, saving Covent Garden from being transformed by the Greater London Council into Alphaville.

Since then, we’ve started listing everything, and Britain has become a museum obsessed with its past. The real turning point came in the 1970s when economic decline slowed the pace of the wrecking ball. “A recession,” Roy Strong whispered to me, “is terribly good for heritage.”

Today a recession is proving good for heritage once more. Cranes have stopped swinging in our cities. Visits to National Trust properties this year are up an incredible 24 per cent. Battles, though, are still raging. The front line these days might be Brutalist bruisers such as Robin Hood Gardens. It might be with the kinds of histories we tell through our buildings. In February, the environmentalist George Monbiot launched an excoriating attack on the cute “tea towel” histories told in too many stately homes, which ignore the hidden tales of land seizure and fortunes made through slavery. One thing’s for certain though. We now know that heritage isn’t so much about what we preserve, but why we preserve it. It isn’t just about architecture. It’s about the people who live in it.

Saving Britain’s Past starts on Aug 24 on BBC Two at 7.30pm

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