Tim Farron is red-faced when he arrives in his sweltering office at Westminster on Tuesday afternoon; he's just back from the gym.

Most people at Westminster are finding the sudden heat-wave uncomfortable but Farron seems chipper, saying he's "very happy" with the coalition and life in general, seeing positive things even in the local elections defeat voters have just inflicted upon the Lib Dems.

What's happened - he says - is that while it looks like there are many Lib Dems who lost their seats, there are fewer council seats overall than in the past - because so many have turned into unitary authorities. And the glimmer of hope for him is that Lib Dems tended to lose seats to Labour, but not to the Tories.

Farron thinks the tax cut for high-earners from 50p to 45p explains a lot; Tories have had that "pinned on them," and although the Lib Dems remain unpopular, they've been successful in making it clear the cut wasn't their idea.

"The Budget itself was actually a good Budget, the presentation was shocking," he says. "Genuinely redistributive from the rich to the poor, the problem is nobody believes that, because 50p is totemic. We said so to the Tories.

"We said the Tories needed to not claim 50p as their price for us getting the income tax threshold increase. We warned them, it would look stupid and it would damage them."

But surely the presentation was so awful because the Budget leaked so ridiculously? "Well, that's because the Lib Dems chose to campaign ahead of the budget for the lifting of the tax threshold. The downside of that was that all the goodies had been made public beforehand."

Although he never says so in terms, it's obvious Farron has decided that the role of Lib Dem party President in a coalition is to function as a steam-valve for unhappy party grassroots members. He describes it as being "a critical friend of the coalition"; in reality it makes him quite totemic himself.

"I can tell you that a Liberal Democrat government would not have introduced academies, would not have introduced the NHS Bill in anything like the shape it began with, probably not the way it ended up either. We would not have had police commissioners. But that still doesn’t mean we’re unhappy. A Conservative government would not have wanted to raise the tax threshold for the least well-off, or kick Trident in the long grass."

But we learned on Tuesday that the MoD is ploughing ahead and poised to award research and development contracts for Trident. "That’s all stuff we knew about anyway," he insists.

Another part of his role - he clearly thinks - is to try to maintain the integrity of the Lib Dem brand. Part of that strategy seems to be about painting Labour and the Tories as similar. "If you were to take the labels off the manifestos of the three parties and say which two have the most in common, you would now have a Tory/Labour coalition," he says. "They are both in favour of making the lowest-paid pay more tax, and they’re both against democratic reform."

He rules out entering the government - "I enjoy being president and I think if I am president I shouldn’t be a minister" - but for quite some time, rightly or wrongly, he's been seen as a potential alternative to Nick Clegg.

So many people have asked Tim Farron if he wants to be Lib Dem leader one day, and like all would-be leaders the answer from him is always no. Farron says it's "distracting and discourteous to Nick."

But does it get wearing, being constantly asked about it?

"I got this at conference - where I’d had quite a week of it, really, " he says. "Nick came up to me and was absolutely lovely, and told me that he’d had all this when Ming Campbell was leader in 2007. Nick had been handbagged by [Ming's wife] Elspeth Campbell for saying sometime moderately ambitious.

"Chances are it was probably Chris Huhne who’d said it, but it was attributed to Nick. Anyway, Nick came up to me at conference and said, ‘Don’t worry, I know you’re not up to anything.’"

How often does he talk to Clegg? "Once a week, sometimes by phone, more often by text. He’s a good texter, very responsive. We probably sit down and have a proper meeting once every couple of weeks."

I ask Farron if Clegg needs to cheer up - it's become a running joke that he always looks sad when he sits next to Cameron at PMQs. Clegg's claim that he was "sad" after the local election drubbing only added to that. Does Farron think his boss needs to cheer up a bit?

"Well he got massive amounts of stick for looking happy, so he stopped that. Poor lad can’t do right for doing wrong. He himself said that the whole rose garden thing, it might have been a good way to set up a new politics, but if the backdrop is horrible financial difficulties and people living in serious fear for their future, looking overly cheerful isn’t good."

Perhaps as a learning experience from the local elections, Farron spends a lot of our interview trying to move the conversation toward attacking Labour, particularly the Blairite period. "If you’re a Tory, we understand. You get into power to do Tory things. I’m cross with Labour. There is a sense that they are like cousins and they’ve dishonoured the family. They not only screwed up their reform package, they left people worse off. I expect it of the Tories, I’m just crushingly disappointed by Labour."

Is there a dialogue with Labour about a potential coalition post-2015? Farron will only say he "talks to Labour people all the time," but he is more open about discussing how the current coalition might break up. He says it's unlikely, but acknowledges that there will be "some kind of process of disengagement, how we will manage that exactly, we don’t know yet.

"We have some blueprints, some models. They are basically Scotland and some of our colleagues over the water in Europe where coalition is more usual. The first time it happens will always be the hardest, all of this is new to everybody."

Could he see the coalition being pared back to something more like confidence and supply? "No, that would involve us walking away from ministerial positions and I think that would be a bit pathetic, really."

I ask him whether, if anything, Lords reform could be a deal-breaker. "The coalition agreement is not something that you can pick and choose from," he says. "If people believe that Lords reform is something that can be dispensed with, for instance, or that it’s tolerable for the Tory whips to not deliver a majority or a diluted Bill, it does bring into a question of trust, and whether other things might be delivered."

Would there be a quid pro quo, with Lib Dems vetoing a key Tory policy if Lords reform founders? "I wouldn’t want to single out particular items that we wouldn’t then deliver," he says.

But the items are there? "There’s a whole bunch of things, there’s three years worth of stuff. The delivery of a wholly or mostly elected House of Lords, in this Parliament, is to be done.

"It’s in everybody’s manifesto and there’s nothing that has more legitimacy," he insists, before turning the conversation back onto Labour-bashing, suggesting they would be adding to their “record of shame on reform” if they attempt to block Lords reform, though he says there is no doubt Labour will try to.

I say how about allowing a referendum on Lords reform - something Labour has now called for? "Labour don’t want a referendum. It’s just something to say. And you don’t have a referendum on something that has universal legitimacy.

"It could be done in a couple of weeks if conservatives stopped whining about it. And when I say conservatives I mean those in the Labour party. The forces of conservatism, to use the Blair phrase."

Finally I ask him how he'll feel if - as some suspect - the Lib Dems could find themselves holding the balance of power in UK politics much more often in this century than in the last one. He gives only a roundabout answer. "It is hard work being in government, much harder work than being in opposition. That’s easy-peasy but very unsatisfying.

"If and when we find ourselves out of power, and it will be when - I’m not predicting exactly when - but we will be in opposition one day, we will miss this. We might not think we will, but we will."