Why the documentary must not be allowed to die

Following the British broadcast, more than 40 sacks of post arrived at ATV's offices in Birmingham, 26,000 first-class letters in the first post alone. Remember this was a time before email and Facebook.

In the letters was £1 million – most of it in small amounts from those who could least afford to give. "This is for Cambodia," wrote a bus driver, enclosing his week's wages. Pensioners sent their pension. A single mother sent her savings of £50.

People came to my home with toys and cash, and petitions for Thatcher and poems of indignation for Pol Pot and for his collaborator, President Richard Nixon, whose bombs had accelerated the fanatic's rise.

For the first time, the BBC supported an ITV film. The Blue Peter programme asked children to "bring and buy" toys at Oxfam shops throughout the country. By Christmas, the children had raised the astonishing amount of £3,500,000.

Across the world, Year Zero raised more than $55 million, mostly unsolicited, and which brought help directly to Cambodia: medicines, vaccines and the installation of an entire clothing factory that allowed people to throw away the black uniforms they had been forced to wear by Pol Pot. It was as if the audience had ceased to be onlookers and had become participants.

Something similar happened in the United States when CBS Television broadcast Edward R. Murrow's film, Harvest of Shame, in 1960. This was the first time that many middle-class Americans glimpsed the scale of poverty in their midst.

Harvest of Shameis the story of migrant agricultural workers who were treated little better than slaves. Today, their struggle has such resonance as migrants and refugees fight for work and safety in foreign places. What seems extraordinary is that the children and grandchildren of some of the people in this film will be bearing the brunt of the abuse and strictures of President Trump.

In the United States today, there is no equivalent of Edward R. Murrow. His eloquent, unflinching kind of American journalism has been abolished in the so-called mainstream and has taken refuge in the internet.

Britain remains one of the few countries where documentaries are still shown on mainstream television in the hours when most people are still awake. But documentaries that go against the received wisdom are becoming an endangered species, at the very time we need them perhaps more than ever.

In survey after survey, when people are asked what they would like more of on television, they say documentaries.

I don't believe they mean a type of current affairs programme that is a platform for politicians and "experts" who affect a specious balance between great power and its victims.

Observational documentaries are popular; but films about airports and motorway police do not make sense of the world. They entertain.