“DON'T worry,” says Ian McDiarmid from outside a Birmingham rehearsal room, “I'm not being attacked.”

The noise from the room next door might easily have given that impression , especially as the Carnoustie-born veteran of stage and screen, former co-artistic director of the Islington based Almeida Theatre and some-time cult hero of the big screen Star Wars franchise, is rehearsing Chris Hannan's play, What Shadows. The play opens at the Royal Lyceum Theatre in Edinburgh later this month in a production revived by Birmingham Rep, who premiered it in 2016. McDiarmid plays Enoch Powell, the old school Tory politician and Wolverhampton MP, who, in April 1968, effectively killed his career when he made what came to be known as the "Rivers of Blood" speech.

The speech, made to the West Midlands Area Conservative Political Centre, a stone's throw from where What Shadows is being rehearsed in Birmingham, was in opposition to the then UK Labour government's proposed Race Relations Bill. Powell never actually used the phrase "rivers of blood", but, in keeping with his scholarly manner, alluded to Virgil's Aeneid, which talked of the River Tiber "foaming with much blood."

Powell may have been sacked by Tory leader Edward Heath the next day, but the speech made him a household name and an icon for Alf Garnett-style bigots. As an illustration of the state of the nation, 1,000 London dockers went on strike in protest at Powell's sacking. An opinion poll found that 74 per cent of those interviewed agreed with what Powell said. What Shadows puts a spotlight on the result of such pent-up frustrations.

“It's a play about how things are in a divided country,” says McDiarmid, “and Enoch Powell is at the centre of it. He's a fascinating character who was the brightest politician of his day, but who was full of contradictions. He had ambitions to be the premier of India and to be prime minister, but he missed out on both. He was the outsider always trying to break through, but he felt it was his duty to give proper expression to the British people. He wasn't a racist, although it's easy to think that from the language of the speech. Chris has written him as a really rounded character, so I don't play him as the Big Bad Wolf.”

While Powell might be play's heart, Hannan also focuses on a second character, Rose. A woman of colour, as a child Rose was traumatised after she heard Powell refer to her and everyone the same colour as her as a "picaninny". Now a successful Oxford academic, she is driven to find Powell in order to write a book about him. As the play moves between time zones, Rose finally gets her wish in what McDiarmid calls “a metaphorical gunfight at the OK Corral.”

Hannan himself likens the meeting to the fragile power-sharing in Northern Ireland between former nemeses Ian Paisley and Martin McGuinness.

“They don't have to love or even like each other,” he says. “They just need to talk, to try and find some common ground so they can work towards finding a peace. I guess the initial idea for the play really was to try and write about identity, and Enoch Powell seemed like a very good way of exploring that. This was about two or three years before Brexit, but UKIP was very much on the rise, so it felt very current, and still does. Ultimately the play is about how to talk to people whose views we hate. It feels like in the fifty years since the speech it's not got better, it's got worse. The whole debate has become so entrenched and toxic, where all these accusations are being thrown at each other, and half the population are being labelled as irrational. This is what we used to say about any race we wanted to put down, and that's crippling the country in a way that it's crippling the Labour Party, where the middle class and the working class are deeply divided. The play becomes about how we talk to each other in a rational way, and the need to find out the rules of engagement to do that.”

For Hannan, who was born in Clydebank and grew up in Glasgow, the initial impetus to write the play came from even closer to home.

“One of the earliest stories my dad tells about me is from when I was three years old, “ says Hannan. “I was being washed in the sink, and this group of boys walked past the window and started laughing at me. My dad asked me who they were, and I said, three protestants and a catholic. How I knew that then shows how much I was being brought up in a deeply divided Scotland. We were brought up with a great deal of grievance, about being discriminated against, and we sang songs about Ireland. There was a lot of hatred there, which lasted quite a long time into adulthood, and then you realise you have to get beyond all that.”

Some of Powell's speech, which was only ever recorded in part by an ATV camera crew, appears in What Shadows.

“Lots of people now will never have even heard of Enoch Powell,” says Hannan, “so I think it's important to hear what he actually said.”

The last time a major surge of anti-immigration sentiment reared its head in a depressed Britain was in the mid 1970s, when extreme right wing organisations such as the National Front adopted a rabble-rousing populist stance. In drama, this inspired Birmingham born playwright David Edgar to write Destiny. First produced by the Royal Shakespeare Company in 1976, Edgar's study of a by-election in a fractured and strike-riven post-colonial Britain arrived in the West End the following summer, at the height of the Queen's silver jubilee fever.

McDiarmid appeared in the Royal Shakespeare's original production as Turner, the radicalised shop owner left politically disenfranchised by Tories and the Labour Party alike.

“I remember there was a lot of picketing by National Front types outside the Aldwych,” McDiarmid remembers, “and David didn't want to see them represented at all.”

Hannan is a fan of the play, and says: “David Edgar was one of very few playwrights at that time dealing with English identity.”

More than forty years on, What Shadows takes its title from a line by Edmund Burke, the eighteenth century parliamentarian regarded by many as the founding father of modern conservatism, and is a very different kind of play.

“The reason I like What Shadows most of all is that there's a strong poetic undertone to Chris's writing,” says McDiarmid. “It's a bit like Powell himself. He was a romantic. People ask me what it's like to be playing Enoch Powell, as if I had to go deep into a dark place to do so, but it's not like that. It's not a play of hard polemic. Yes, it's political at its core, but there are other resonances and other colours at play.

“It's a play about the national debate now and then, and people watching it will take sides. The problem now is that people are so bloody angry that they're not capable of having a rational conversation, and if you want to move forward you've got to be able to do that.”

What Shadows, Royal Lyceum Theatre, Edinburgh, Thursday to September 23.

www.lyceum.org.uk