There are a lot of characteristics that seem important to our conception of Nelson Mandela. Patience. Wisdom. Fortitude. Being South African.  

One that doesn't make that list is his height. Yes, Mandela is a big guy – 6ft 4in, to be precise. But he is also the defining figure of the late 20th century, one of the few world figures who properly merits the term hero. When you think of Mandela's defining qualities, his height is way down the list. His South African-ness, on the other hand, once we get past the superhuman stuff, is pretty intrinsic.

Moonyeenn Lee, the casting director for an adaptation of Mandela's autobiography Long Walk to Freedom, sees things a little differently. Lee auditioned some South African actors for the part, but in the end he went for Idris Elba, the British-born star of Luther and The Wire. The South Africans were "extraordinary", he explained at the weekend, but in the end they were just too little. Only Elba had the requisite stature. And so the part was his. The decision adds Elba's name to a list of luminaries – Sidney Poitier, Danny Glover, and Morgan Freeman among them – who've had the honour. But as yet, the part has not been played by a South African.

I did briefly entertain Lee's argument. I wondered if a South African man of 5ft 7in would be utterly implausible as a South African man of 6ft 4in. Then I remembered that Elba is covered in muscles, rather unlike the slight-framed Mandela. I also thought of Ben Kingsley's turn in Gandhi, and Cate Blanchett's Bob Dylan in I'm Not There, and the human-sized Ian McKellen's ability to portray an eight-foot bloody wizard in The Lord of the Rings, and I concluded: perhaps – perhaps – Lee isn't telling the whole truth. Perhaps he made a more cynical calculation.

Such complaints will inevitably be dismissed as carping by some. They might point out that Meryl Streep was an almost universally popular Thatcher, that Blanchett's Elizabeth I was just as uncontroversial. But neither Thatcher nor Elizabeth quite defines Britain's sense of itself in the way Mandela does South Africa's. And it must be admitted that whereas myriad Brits have played both those roles, an unbroken string of at least eight prominent foreigners playing Mandela without a South African getting a look-in seems too much to be a coincidence.

The sad fact is: you can understand Mr Lee's real dilemma, distasteful as his excuse might be. He wants people to come to the movie. He knows they respond to a star. But Elba is hardly an A-lister; what will sell the film to the audience is Mandela's extraordinary story. What a shame his fellow countrymen have once again been excluded from telling it.

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