Imagine: Dame Shirley Bassey, BBC One

The mechanism for securing a publicity still from the BBC is as follows. Go to the relevant website, log in, look for the photographs that illustrate the programme, then take your pick. For Dame Shirley Bassey: The Girl from Tiger Bay there wasn’t much of a selection. Only one image, in fact, at least that I could see. It finds Alan Yentob perching like a prize-winning schoolboy on the edge of the sofa, while the prize leans intimately on his shoulder.

We need Imagine, especially now that the guillotine is slowly and insubordinately lopping Lord Bragg’s coiffeured head off over on the other side. But really, is a little distance between interviewer and subject too much to ask for?

A photograph of Burly Chassis with more or less anyone else featured in this documentary would have been just the ticket. The Dame has come out of retirement to record an album of songs tailor-made especially for her by songwriters as diverse as Gary Barlow, John Berry and Don Black, KT Tunstall, the Pet Shop Boys and the Manic Street Preachers, all of whom piped up to pay their respects. They don’t make singers like her any more, explained Barlow. “That’s probably why everyone’s been so excited about it,” he added, sounding unexcited as only he can. But apart from some stolen shakycam footage of Barlow introducing her to her bespoke song, Imagine signally failed to get any of them into the room with her.

The aim of these project-access arts documentaries is to reveal the artist at work, and tell the story of the life and times along the way. Bassey, however, is almost catatonically uninterested in revelation. “You are a very private person, aren’t you?” said Yentob, presumably having long since signed away the right to ask her about having a child at 17, the two divorces, the death of her 21-year-old daughter, and above all about the rare and intriguing circumstances of her childhood.

The daughter of a mother from Yorkshire and father from Nigeria, Bassey was the youngest of seven mixed-race siblings growing up in Cardiff’s working-class docks in the 1940s. There must have been a sense of alienation as the lone dark face in the school photograph, not to mention as the family afterthought whose father disappeared when she was two. She did remember singing in bed to cheer herself up, but that was the only flash of thigh she was going to show Yentob. “I suppose I should be analysed,” she mused towards the end of the film, parked in her Monaco apartment with walls plated in gold discs. Just not on BBC One.

What ended up being fascinating about this portrait was the fact that her songwriters were left with a blank page to work with. And every time they seemed to intuit right. The Manics served up the allusive title song having failed for years to get Tiger Bay into one of their own compositions. Barlow explained musicologically how he wanted to marry the Bassey larynx to the interval of a sixth beloved of Bacharach and the Beatles. He also wanted to know if she can still sing across three octaves. Neil Tennant more conservatively chose to restrict himself to a range of about four notes. She performed his song, “The Performance of My Life”, at the BBC Electric Proms last month, and while melodically it’s a dirge the confessional lyric – featuring an uncanny lexical overlap with “My Way” - gave this careful homage an unexpectedly moving finale.

“It’s like they got into my head,” she said of her composers. “How do they know so much about me?” Guess it takes one to know one. Which is why Yentob didn’t get the story. The most revealing shot of the film found Yentob with his ear glued to the door of a rehearsal room, listening. “Dame Shirley is in there,” he whispered. Never has a door stayed more firmly closed. That’s why the publicity still is so misleading. Boy, did she not cry on that shoulder.

Watch The Girl from Tiger Bay on BBC iPlayer until 22 December. Buy The Performance.