If it could be heard in Westminster, there would be two reasons for him to press it. Firstly, he wants the government to fast-forward all attempts to cure Parkinson's disease, from which he suffers. "The saliva and shakes make me seem gaga" the 78-year-old says of his disease. "I'm not. I remember pretty much the lot."

"The lot" is the other thing for which he'd like attention. Thorpe has firsthand experience of coalition talks of the kind Nick Clegg may find himself in if the next election produces a hung parliament, as some predict.

Like Thorpe before him, this would make Clegg a kingmaker, and that's why David Cameron and Gordon Brown both rang Clegg on his election. They are already pawing at his ermine.

"I'm sending them [the party leaders] a memorandum on constitutional reform. To concentrate their minds," Thorpe says, words spoken through a veil of Kleenex held to his mouth by an unsteady hand. "I still have steam in my pipes."

Thorpe was a youthful leader of a political party in the days before it was feasible for Richard Curtis to cast Hugh Grant as the PM.

From the age of 38, he led the Liberals for nine years, flaunting his advantage when, out on the election trail in 1974 wearing a trademark trilby, he vaulted a security barrier like a Moulin Rouge can-can dancer. Ted Heath would have had the barrier moved; Harold Wilson would have taken out his tobacco to think about it. "I'm not overly concerned by Clegg's youth," Thorpe says.

The full head of Brylcreemed black hair has kept its hairline and faded to sepia, not white, but although a swivel chair allows Thorpe energetic spins to and from his desk to plunder the box of tissues, his posture is stooped and his body rigid.

From 1967 to 1976, surviving a dire performance in the 1970 general election, he turned the Liberals from a party small enough to squeeze in a taxi (six MPs) into a party small enough to fit in a minibus (11).

In the 1974 election, the Liberals won 19% of the vote (then a post-war record) and got 14 MPs. Although Ted Heath hadn't won a majority, he had won the popular vote and refused to resign. Thorpe went to Downing Street for coalition talks with Heath, whom even the Spectator was calling a "squatter" in No 10.

Although the quickfire decision on compatibility in these talks - both pro-European, "some similarity on statutory incomes policy," Thorpe recalls - was speed-dating rather than true love, and although the coalition would have needed the cooperation of another group such as the Ulster Unionists, it was, and remains, the closest to actual government the third party has ever come.

"Both the Tories and Labour will be prepared to pay a high price for office," Thorpe says, pausing every handful of words. "I think the Tories are desperate, defeated twice already. And they are ruthless beasts. Labour is also likely [to want a coalition]. They will want to hold on to power."

He has noticed that until very recently Scotland was ruled by a Lib-Lab coalition that lasted eight years. And in local government, Lib Dems are on councils with Tories - coalitions of sorts. So is a Westminster coalition inevitable?

"We're not very good at coalitions in this country," Thorpe says. "Voters get suspicious of [coalition] because it means you're in hock to someone else."

But coalition delivers power, doesn't it? He shakes his head. "I'd prefer Clegg not to go into coalition. He should keep the party a free agent, keep on opposing the government."

Here, half a lifetime later, Thorpe admits committing a mistake when he eventually took Heath's phone call (an irate PM only got through to Thorpe at bedtime on the day of the election).

"What I should have done is ask him: 'Why do you want to meet? Is it about coalition? Because if it is, it's not on.'" Liberal activists hated the idea of propping up Heath. A former chairman of the Young Liberals, Peter Hain, warned of mass resignations from the party. They had not, to adapt David Steel's later rallying cry, "gone back to their constituencies to prepare for coalition".

What of Cameron's recent offer to Clegg to build a progressive consensus? He slumps a bit. "I would be very shocked. Our role is not for the Tories."

Thorpe's constituency was naturally Tory and, like Clegg today, so were many of his target seats. Although Thorpe did agree with Heath on Europe - "without the Liberal vote they wouldn't have had the majority [to get into Europe]. On that we saved the government" - for him and Clegg, the Tories are the opposition. "Cameron is a phoney," Thorpe says of his fellow Etonian. "A Thatcherite who is pretending to be progressive."

And what of rescuing Brown? Thorpe doesn't rate the man: "Dour and unimpressive. Like Callaghan ... You have to go back to Attlee to get a really good Labour PM."

"We are to the left of the Labour party on quite a few things," he says. "Brown has some very Tory views. He's just as responsible for Iraq, clinging on to Bush's shirt-tails."

Thorpe's father and maternal grandfather were Conservative MPs, but he became a Liberal because, he says, of international blunders by the Tories (he cites Chamberlain's policy of appeasement and later the party's Suez "adventure"). Of course, he hates the Iraq war.

He also warns about the practicality of a coalition with Labour. "There's a significant proportion of the Labour party - 70 or 80 MPs - that are so disillusioned with the leadership of the party that they'd prefer to be in opposition than in coalition. I think they wouldn't stomach a Lib-Lab coalition. So it would fall apart."

Wouldn't all this be academic if the Liberal Democrats were granted what they've always wanted: proportional representation? PR was the deal-breaker in 1974 - wouldn't it be a deal-breaker now?

On hearing Thorpe's PR demands, Margaret Thatcher is supposed to have said: "Oh no, we couldn't. Think how many votes we would lose." Thorpe is pessimistic that either of the main parties in Westminster now think any differently about PR. "Our role is to provide a powerful opposition group, preferably outside government," he repeats.

He also offers something of the advice he will send party leaders about PR. "In the large rural areas, enormous constituencies, it is better to keep a single MP." PR should be saved for the cities, he says - a revision from a one-time proselytiser.

Thorpe's coalition cynicism might also be because, in the second election of 1974, the Liberals lost an MP. "People got frustrated by the time of the second election - 'what's the point of voting for the Liberals?' So they stopped," he says.

Maybe the scars of 1974, rather than the realities of 2008, inform Thorpe more, but Clegg will bear in mind the unpopularity of coalition talks with the grass roots.

"Instead, Clegg must win as many by-elections as he can." Thorpe taps the arm of his chair as he lists the five by-elections his party won under his stewardship. How can Clegg do this? "I had an uncanny way of finding out who was going to die," Thorpe says. "Instinct. I gave Richard Wainwright [a Liberal MP] a tip and he said: 'Is he dead?' ... I said: 'No, not yet, but he's going to ...' And he died."

There is advice aplenty - but when Clegg was elected as the Lib Dem leader last month, a reunion of past leaders was held and Thorpe was the only one not present and, it seems, not invited.

Possibly it was Thorpe's fragility. Possibly it was what Matthew Parris has described as the "most sensational [scandal] of the parliamentary century, bar the Profumo scandal".

Two years after walking up Downing Street, Thorpe resigned as leader of the party after being accused of conspiracy to shoot a man, Norman Scott, who claimed to be a former lover. Scott had been out walking a dog and, though he survived, the animal was killed. Thorpe was acquitted on all charges in 1979, but had by then lost his seat and his party.

"If it happened now I think ... the public would be kinder. Back then they were very troubled by it," he says. "It offended their set of values."

Harold Wilson thought the allegations a Conservative smear, asking in a memo to one of his ministers, Barbara Castle, why damaging details surfaced later in the 70s at a time when Labour might want to go into coalition with the Liberals, rather than earlier when Heath wanted them.

There are supposed to be new details that will emerge about the Thorpe trial only after his death - all he will say now is that it was more likely to have been his opposition to apartheid that brought about the trouble. "South Africa certainly attempted to smear me. They made life very difficult.

"I wanted to clear the air, but I was pretty shattered. I would have gone on leading the Liberal Democrats. I think I could have pushed up our number of seats."

Thorpe sees himself as a guardian of the Liberal vote through difficult times. "It's now a three-party system and I think I built it up. I know it's an arrogant thing to say but, if it weren't for me, the party would have disintegrated."

Instead, Thorpe wanted a government of national unity - with himself, Wilson and Heath all cooperating in one big tent. It remains to be seen whether, if faced with a hung parliament and coalition talks, Clegg will think that two's company and three's a crowd.

· This article was amended on Tuesday January 29 2008. We meant to say 'in hock', not 'in hoc', in a quote from Jeremy Thorpe. This has been corrected.