I f you want a quintessential “It wouldn’t happen now” story, try this. Two months before Margaret Thatcher became prime minister in May 1979, a national Militant Entertainment Tour was organised by Rock Against Racism .

On 20 March 1979, it came to the West Runton Pavilion near Cromer, the (very) small town on the coastal edge of East Anglia. The bill mixed the black and white musicians in three bands then at their artistic peak – the post-punk maestros Gang of Four, the punk-reggae hybridists The Ruts, and Misty in Roots, the renowned reggae collective from Southall in west London – and thereby made a stark symbolic point: that the multiracial nature of large swathes of the UK was not to be decried or attacked, but actively celebrated.

The UK was then – and some of this may sound familiar – in a bitter, polarised state, seemingly locked into deep economic problems, amid rising fear about the neo-fascist National Front.…