“Men are apt to mistake the strength of their feeling for the strength of their argument” observed William Gladstone. “The heated mind resents the chill touch and relentless scrutiny of logic.”

Natalie Bennett has experienced that chill touch in what has inevitably been dubbed a “car crash” interview with Nick Ferrrari on LBC this morning.

Amid long, deathly pauses, the Greens’ leader couldn’t explain how she would fund the 500,000 new social houses the party is committed to building.

Commendably short on spin, she later described her performance as “absolutely excruciating”.

Her strategic mistake was to even try.

Although the Greens, like UKIP, have no realistic prospect of forming a majority government in May, they have fallen into the trap of accepting the burden of proof expected of the main parties who do hope to.

So Bennett shoots for financial credibility and misses. Meanwhile, Nigel Farage cracks down on UKIP’s red-trousered eccentrics for saying the ‘wrong’ outrageous things. They shouldn’t bother. They are succeeding despite their obvious flaws.

We may expect ministers and their shadows to have detailed policy on everything from agriculture to youth services, but is there anyone considering voting UKIP or Green because of their views on apprentices, or business support grants?

They are what we used to call single-issue parties, representing, to borrow Aneurin Bevan’s phrase, “an emotional spasm”. People aren’t voting for them because of their realism.

They are a release valve for those who are either terminally disenchanted with the mainstream or are well-off enough to avoid the appeal of pocket-book politics and let their cross on the ballot paper reflect their “post-material values”.

UKIP is home for those who rail against the dying of the light as Johnny Foreigner’s jackboot looms over this scepter’d isle. By embracing low-fi political correctness and reacting to media stories about the endless gaffes from its candidates, UKIP undermines its essential raison d’etre.

Similarly, the Greens’ anti-growth, anti-car, hemp-shirted idealism chimes with well-educated urban trendies who don’t like Labour and are turned-off the Lib Dems. Their appeal is not going to grow because Natalie Bennett suddenly embraces fiscal rectitude.

The smaller parties are niche vehicles for protest voters. They are not going to sweep the country in May. The best they can hope for is to localise their support in enough places to make a bridgehead.

The danger for them is that by playing to the big boys’ rules, our smaller, newer, woolier insurgents will get found out in the intense glare of a general election campaign. They need to keep their offer simple.

What Natalie Bennett should have said this morning is that the Greens want a law compelling central and local government to work in partnership to plan and provide enough social housing to meet need.

Simple, rhetorical and internally coherent enough to bluster through a radio interview.

“Yurts for all,” so to speak.