Norfolk; the bleak midwinter of 1938. On the county’s eastern fringes, the North Sea has demolished coastal defences and flooded inwards, turning farms into islands, marooning villages and contaminating thousands of acres of prime arable land. Refugees line the roads, driving horse-drawn carts laden with whatever possessions they can rescue. And on a waterlogged country lane, Claude Simmonds and Bob O’Brian stop their post van, pull on their wellies and climb into a rowing boat bound for the village of Horsey. East England may be in chaos, but the mail still gets through.

Captured in grainy black and white with a crackling soundtrack, these moments provided some of the opening scenes for The Horsey Mail, a nine-minute documentary made soon after the 1938 floods had struck, while much of Norfolk was still underwater. Its creators worked for the General Post Office (GPO) itself, and the film is one of dozens now being analysed afresh in the first ever history dedicated entirely to the organisation’s pioneering filmmaking arm.

Launched this week, The Projection of Britain follows the story of the GPO Film Unit from its birth in 1933 through to the post-war era. With contributions from film experts, historians, literary critics, film archivists and a postman, among others, it seeks to show how what was conceived as an offshoot of the post-office’s public relations department evolved into an experimental hub, and one of the founding pillars of British documentary.

It also reasseses the battered reputation of the Unit, originally invented by the GPO (forerunner of today's Post Office) to shore up its own image. In recent years, GPO films have frequently been ridiculed for their RP voiceovers and dated, sometimes wooden style. Often they are seen as the parochial relics of a naïve past, short on realism and in thrall to the establishment. One 1970s critic even likened their perspective to: “a Greek scholar-cum-brigadier… occasionally peeping at his loyal lads through the wrong end of his opera glasses.”

Not so, the new study claims. GPO films may look like natural ammo for Harry Enfield’s Mr Cholmondley-Warner, but closer scrutiny suggests that they are far from stiff upper-lip. In fact, these documentaries introduced a new level of realism to British cinema and represented the lives of ordinary working people in a manner previously unseen. While World War II is often regarded as having “democratised” film in this way, the book’s authors believe that the GPO was doing it long before the war had even begun.

“The GPO film unit sat at the creative epicentre of 1930s Britain, but people rarely write about it as a place where cultures met,” Dr Scott Anthony, a Cambridge University historian and one of the book’s co-editors, said.

“Instead, people have tended to see them quite narrowly – as little more than a staging post in the development of documentary film. The film unit’s output should be seen beyond that and in the context of its time. It is interesting in every direction; aesthetically, socially, culturally – and politically.”

The film unit was established in 1933 by Sir Stephen Tallents, the GPO’s head of public relations. At the time, the post office was one of Britain’s biggest employers, with a quarter of a million staff on its payroll. Films like The Horsey Mail were meant to tell their story and that of the GPO’s role in wider society. They were screened as shorts before main features at the cinema, in communities around Britain courtesy of mobile cinema vans, in schools, and at meetings of social organisations like Trade Unions and women’s groups.

What might sound like an exercise in corporate advertising, however, was really something far more subtle and complex. Tallents, in a 1933 pamphlet, The Projection of England, had laid out a vision for a film school that would enable Britain to develop an honest cinematic image of itself, able to fend off negative sterotypes found in the films of fascist Germany, Soviet Russia and the United States. He also wanted to bridge the gap between ordinary people and the establishment represented by government and national institutions. At a time when state services were growing, he focused on humanising the state, using film to educate its bureaucrats to respect a working population hit hard by the slump.

At the head of the film unit was the Calvinist-inspired founding father of British documentary, John Grierson. He believed that films should have a “socially useful purpose” and was inspired by the achievements of Soviet filmmakers, led by Sergei Eisenstein, who had helped to engender that sense of unity by reflecting the lives of ordinary working people.

Under Grierson’s stewardship, the unit assembled an ambitious and imaginative collective of international artists, including Lotte Reiniger, Norman McLaren and Len Lye. Given relatively free rein by Tallents, they experimented with sound, speech and subject matter. The films they produced (both with and sometimes without the GPO’s official blessing) reflected the full gamut of working Britain, chronicling the lives not just of post office workers, but coal miners, fishermen, and people living in its slums.

In the context of the time, this was a political statement in its own right. Mainstream cinema was doing little to portray the realities of British life. When it did tackle issues like inequality and social unrest, it did so in a sanitised, jingoistic way. The GPO film unit was different. It was a form of public service cinema representing wider social concerns. Those sponsored by the British Commercial Gas Association, for example, aimed to sell gas cookers and heating – but they did so by raising problems of working class deprivation: Housing Problems (1935) highlighted the state of Britain’s slums; Enough To Eat (1936) addressed questions of malnutrition within them.

Bigger statements still were made elsewhere, often about the unifying and democratising force of new technologies that were being pioneered by the GPO. In the “film talk”, We Live In Two Worlds (1937), the playwright JB Priestley juxtaposes the “limiting and quarrelsome national world” with a universal future of technology and personal telecommunications that transcends national boundaries. It’s an excellent demonstration of the fact that far from being dated, the films of the GPO Film Unit often speak directly to the concerns of today.

In this way, the book encourages critics to look again at GPO films. Its most famous film, Night Mail (1936) - a love letter to the postal express soundtracked with a poem by WH Auden and a score by Benjamin Britten – seems like a straightforward documentary about how the post gets from King’s Cross to Edinburgh. On the way, however, we see the lives of ordinary working people and are shown how the combined forces of the GPO and the rail networks connect them regardless of wealth or class. The films often stress the interdependence of all British citizens. When Claude and Bob pull on their wellingtons in The Horsey Mail, they are likewise working class heroes cheerfully doing their jobs in the face of adversity, two full years before “Blitz Spirit” became a byword for the same pluckiness in harsh times.

For these reasons, Anthony and his co-authors believe that it is time to end the dispute about whether the GPO film unit deserves to be recognised as an essential part of British cultural heritage. Their book also argues that the collection provides a vision of a now almost forgotten past. The recent discovery of the films of Blackburn pioneers Mitchell and Kenyon showed a lost world of life, work and play in Edwardian Britain. Similarly, the GPO archive refers to a now distant time. The steam-driven postal express of Night Mail races past mines in Wigan, steelworks in Warrington, machine shops in Preston, on towards Glasgow’s thriving shipyards. Like the Edwardians, its world has also now disappeared.