Watching Hugh Grant’s TV portrayal of Jeremy Thorpe, it is almost impossible to believe that such an extraordinarily reckless public figure could really have prospered in 20th-century British politics. But he did.

The insouciance, the exhibitionism, the darkness and the utter unreliability that Grant captures so brilliantly in A Very English Scandal may seem like a grotesque caricature of Thorpe. But it isn’t.

Forty years on, and especially in the social media age, it seems inconceivable that a major politician could have led a double life as a promiscuous gay man and a pillar of the parliamentary and social establishment, without his colleagues cottoning on. Especially when he tried to have his former lover murdered. But he did – and they didn’t.

Did you know at the time that Thorpe was gay, I asked his successor as Liberal leader, David Steel, this week. Steel’s response, speaking from his home in Scotland, was instant, vehement and almost astonished. “No. Absolutely not. It was a surprise when it all came out.”

On paper, Thorpe was a mid-20th-century Tory politician from central casting. Male, white, Eton and Oxford, son and grandson of Conservative MPs, socially well-connected, brought up in Knightsbridge in a house with a cook, chauffeur, four maids and a nanny. But an admiration for David Lloyd George, whom Thorpe met several times as a boy, led him early into the Liberal party.

“He was a very substantial radical,” Steel recalls, “especially on foreign policy, but also on issues of individual liberty. I admired him rather than liked him, but he was very charming, always fun to have around. He was not someone you warmed to. But until the end everyone was very loyal to him.”

As well they might have been, given Thorpe’s early achievements as Liberal leader. When he succeeded Jo Grimond as leader in 1967, the party had 12 MPs and had won 2m votes at the previous election. Seven years later, in February 1974, Thorpe tripled the Liberal vote to more than 6m, though still with only 14 MPs, coming close to forming a coalition with Edward Heath’s Tories. But hubris lay just around the corner in the shape of his trial for conspiracy to murder his former lover Norman Scott.

As one might expect of someone who is portrayed in A Very English Scandal, Steel has been hooked on the series, the final episode of which will air on Sunday on BBC One at 9pm. “They have obviously compressed a lot of the story,” Steel says, “But the basic thrust of it is right. It’s reasonably accurate in most respects. And Hugh Grant is genuinely remarkable.”

Months ago, Grant got in touch with Steel to ask him for advice on the kind of person Thorpe was. The two men met at the House of Lords – exactly the sort of grand setting that recurs so plausibly in the series. During lunch in a dining room overlooking the Thames, Steel reminisced to Grant about the events of the 1970s and gave the actor some tips about Thorpe’s character and quirks, his ways of talking and behaving. Steel is delighted with the result. “Uncanny,” he says of Grant’s performance.

Steel also has his gripes, though they not political ones. Mainly they are about the cars that Thorpe drives in the series. “Jeremy drove a Humber Super Snipe, but they showed him in a Rover 3 litre. And in the second episode Thorpe is driving a white Triumph Stag when he sees Scott again. That should have been a white Rover 2000.”

“That scene where I interview Scott at the House of Commons isn’t right either,” he continued. “Emlyn Hooson [played by Jason Watkins] wasn’t actually there at all. He had another commitment and asked me to stand in. It was just me and Scott. I went into the meeting thinking that Scott was going to complain about Peter Bessell [played by Alex Jennings]. It was only during the meeting that it became clear he was talking about Jeremy.”

Steel may have had a ringside seat at some of the political events as Thorpe fought to save his doomed career, but he is certainly not the only former Liberal who has been glued to the TV the past two Sundays and will be again on Sunday. “I’m pretty sure all Liberals are watching it,” says Paddy Ashdown, who succeeded Steel as leader and who was an aspiring MP during the Thorpe scandal. “But I think some of them have been dreading it.”

It is true that some of those old enough to remember Thorpe or who have connections with the real people depicted in the series, have been worried by the portrayals. Norman Scott, still living in Devon at the age of 78, could be one of them. But, just as Grant talked to Steel, so Ben Whishaw sought out Scott before the series got under way. Scott was worried that the series would be “a second trial”, but according to the Radio Times last month he was “very moved” by the results. “He was very pleased, he laughed and cried,” reported the director, Stephen Frears.

Alex Carlile, who followed Hooson as the Liberal MP for Montgomery, is less pleased. “I thought that the portrayal that [Watkins] was forced to give by the script was rather unfair to Emlyn,” he told the Shropshire Star. “He was portrayed as devious and conniving whereas in reality he was extremely frank and never underhand.” Carlile nevertheless joins enthusiastically in the chorus of praise for Grant’s portrayal of Thorpe.

“We looked on with incredulity,” recalls Tom McNally, now a Lib Dem peer but from 1974 to 1979 political secretary to Labour’s Jim Callaghan as foreign secretary and prime minister. “I sat in on meetings he held with Jim in the 1970s. He was very funny. A great mimic. But I was never remotely aware in any way of what was happening in his private life, and I’m not aware that Jim was either.”

Thorpe’s career never recovered from the trial, at which – spoiler alert – he was acquitted in 1979. He lost his parliamentary seat the same year. Developing Parkinson’s disease, he died in 2014. For a while, he haunted party events. Almost to the end, he would attend memorial services for former colleagues.

“He spent 30 years trying to persuade every successor as party leader – right up to Nick Clegg – to nominate him for the peerage he craved,” Steel recalls. But the peerage never came and was never going to come.

In his final years Steel visited Thorpe in his house in Orme Square, Bayswater. He was physically weak but still focused. “He was difficult to understand. He spoke through a little microphone thing.” Ashdown also recalls meeting Thorpe at a memorial service during those bleak later years. “I put my arm around his shoulders. He was just a bag of bones.”

A Very English Scandal concludes on Sunday at 9pm on BBC One