The most moving documentary I have seen on British television is broadcast tonight. Letting Go is a portrait of the ordeals, anxieties and joys of families containing children with disabilities.

It is presented by Rosa Monckton, whom I have met once and whose husband, Dominic, is a friend. Their daughter, Domenica, has trisomy 21, also known as Down's syndrome, and recently reached her 16th birthday.

This fact occasions a reflection on how hard it is for all parents to let go of their children but especially when the child has a disability. Fewer than one in five people with disabilities gets a job; most of those are unpaid. Living independently is basically impossible for many, though parents are always tempted to give their children a taste of adult life. We are introduced to Richard, who has Down's syndrome. Six years ago he moved into a flat near his mother, Dawn, but needed 16 hours of support each day and was bullied by his neighbours. Then he moved back. Dawn says that in some ways she would rather her son died before her: that way she at least knows everything will be ok.

The parents of Jess Hiles, 28, are amicably separated. Her condition is a 98 per cent match to something called Williams syndrome - but because of the two per cent shortfall, it's extremely difficult for her to get funding. Without a clear diagnosis, her parents' lives are unbearably traumatic. There is an unwatchably sad moment when Rosa asks Jess if she can imagine life without her parents, and she breaks down in tears.

There are hugely uplifting moments, too, such as the scenes from an extraordinary charity called Chickenshed, which encourages children with disabilities to get into the performing arts. This is one of the pillars of the argument that Rosa builds up over the show. For children who spend so much of their lives feeling excluded, the sense of joy in environments where they are happy, can express themselves, and feel deeply included is hard to beat. We need to create many more such environments.

We also need to end the absurd discrimination against those with imprecise diagnoses, which is the exact opposite of what is morally necessary. And the trauma of having to persuade local authorities, who ultimately provide much of the funding for carers, that this boy or that girl should be allowed to leave home is a burden these parents can do without.

Ours remains a society in which fear of the otherness represented by disability remains a profound and widespread vice. Letting Go (on BBC1 at 10.35pm) challenged my preconceptions, and alerted me to everyday horrors and heroism which I knew nothing of. I suspect you'll be moved, too.

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