Farron, 44, is still yet to officially announce he is standing for the leadership of the party. Yet he has long been the bookies' favourite to take over from Nick Clegg, who stepped down last Friday.

Sitting on his window bench in the sun, Farron praises Clegg for "playing a blinder during the election campaign".

"We went into coalition with our eyes open," he says, "and while I may disagree with one or two things we did, nevertheless I think we did the right thing going into government. I'm very proud of him. He went in doing the right thing by the country knowing it was going to damage him personally."

The former party president appears affable and relaxed – until he talks about the fate of his colleagues, when he leans forward into the room to press his point, visibly pulsing with emotion.

"There's a sense of horror and outrage," he says, "that the party that's done so much good over five years was, by an incredibly well-funded tsunami of negativity – directed not at us but at either the English nationalists or the Scottish nationalists – swept away in the middle."

He insists he has no regrets about going into coalition. "History will be very, very kind to Nick Clegg, kind to the Liberal Democrats these last five years," he says. "Which makes me a bit angry – why didn't you give us our due when we needed it?"

However, he does promise that the Tories – whose route to power lay through the near-obliteration of their coalition partners – will pay the price.

"I do feel angry," he admits. "I mean, the sense is they will get their comeuppance. I'm certain of it, because the last thing David Cameron needed was to have a small Tory majority and a European referendum. It's quite possibly the end of the Tory party. This is either 1992 or 1974 and either way it's bad for the party in power."

As for Labour, he professes to have very little time for them either. "You've got a Labour party that doesn't have any narrative whatsoever," he says. He adds that they "cannot cope with an admission of some level of guilt in terms of getting the country in the mess it was in. [They're] an illiberal party, a party that tends to use UKIP-ish language on immigration.

"But I think in one sense – when you've got eight MPs, it's hard to argue this – but the Labour party are bit players in all this. They're not engaging in the main debate, I'd say we are."

He gains comfort from how former leader Paddy Ashdown turned around the party's fortunes after the "debacle" of the 1988 merger between the Liberal party and the Social Democratic party.

"I saw how Paddy Ashdown got the party by the scruff of the neck, gave it vision, gave it belief, set up an infrastructure, and just had an infectious bloody-minded collective determination that we were not going to die and we were going to do quite the opposite," he says.

"Within less than a decade of him taking over we got the best number of seats we'd had since Lloyd George's day."