Why hours of media training have proved worthless for the leaders

For the past five months, they have been schooled on establishing the correct "eye line", coached on posture and in deploying such debating tactics as "the bridge", "the power of three" and the "riddle and solution". They have rehearsed when to go on the attack and when to introduce a joke or anecdote.

But now that it comes to the denouement, the last of the three historic televised election debates, it appears that all of the media training that the three party leaders have undergone since the contests were agreed on 21 December may have been a waste of time.

Media trainers spoken to by The Independent were largely of the opinion that the Conservative leader, David Cameron, a former television industry communications professional who was previously widely admired for his debating skills at the dispatch box, has underperformed after being over-prepared for the cameras, his slick image overshadowing his real personality and failing to convince voters.

Nick Clegg, widely seen as the star performer over the opening two debates, was seen by the trainers as being simply fortunate to be a "natural", possessed of instinctive broadcasting skills that would shame experienced journalists on national news bulletins.

As for the Prime Minister, after all the long rehearsals designed to minimise the risk of a gaffe, he makes a blunder that would have embarrassed a teenager on work experience at a hospital radio station. The Prime Minister's on-microphone denigration of Gillian Duffy on the eve of the final debate will have appalled his media advisers and means that he goes into the encounter with a self-imposed disadvantage.

Lindsay Williams, a former Radio 4 journalist and managing director of The Media Coach, said the Prime Minister would be ill-advised to make reference to the matter at the start of the debate, broadcast this evening by the BBC. "If he was my client I would say, 'You've got to be prepared to address it, but don't raise it,'" she said. "It's in his interests for the world to believe that it's a minor under-pressure gaffe. But you would expect his political opponents to make political capital out of it, so he has to be prepared."

Mr Brown has more than the Gillian Duffy affair to worry about. "[John] Prescott keeps saying that Brown is a very warm and witty person, I don't think any of us have ever seen that," said Ms Williams. The Prime Minister's attempted jokes, crudely inserted into the discussion, have repeatedly fallen flat. "He has used humour very badly," said Nici Marx, a former anchor for Sky News and CNN International. "When he comes out with something that's supposed to be light-hearted it's backfiring, it makes him come across like a schoolmaster."

Susan Bookbinder, a former newsreader for Radio 5 Live who runs the Zamala Media Consultancy, said that because Mr Brown had been instructed to smile, his grin appeared as a "false veneer". She said the Prime Minister could only convince voters if he behaved authentically, even if that meant showing his anger. "[One] thing that lets Brown down is that he's got this dwarfish stature – he needs to hold his head up high, get the neck up out of the collar and be himself, and if that's angry then be angry."

Tonight's debate, from a venue in the Midlands, will see Mr Clegg take the position at the central lectern, following the drawing of lots by party representatives. Mr Brown will remain on the right of the screen because he has lost sight in his left eye.

Ms Bookbinder said that the Prime Minister's visual impairment was no excuse for his failure to look at the camera. "I have coached people to have good eyeline even if they've got one eye. It is absolutely critical. If you are not prepared then your eyeline will wander off as you access your memory or the creative side of your brain, thinking on your feet. The audience doesn't know the difference and thinks 'This person is lying'."

Another veteran BBC journalist, John Venables, managing director of NewsCrews, said Mr Brown's position was already hopeless. "As a trainer I would struggle to deal with Gordon Brown whatever his message because he's starting to look really haggard. Will the audience want somebody who looks like yesterday's man and who looks exhausted?"

The former Chancellor of the Exchequer will hope to score points through his knowledge of financial affairs. "The first 45 minutes of this debate is going to be on the economy and that is going to feel different, it's obviously one of the most critical areas in the election campaign," said Sue Inglish, the BBC's head of political programmes.

Even the experts have been surprised by the under-achievement of the Tory leader in the first two debates. Both Mr Venables and Ms Marx said Mr Cameron had resembled a second-hand car dealer. "Unfortunately he looks rather too slick, polished and urbane," said Mr Venables. Ms Marx said she would try to address an irritating mannerism. "He still does that pursed lip thing, the lizard tongue. He does need to watch that because it gives the impression of being quite petulant."

All were agreed that Mr Cameron – who has the opportunity to raise the Duffy affair when he opens tonight's proceedings – must be more assertive and statesmanlike, showing passion without picking petty quarrels. And he should drop the catchphrase "If I was to be your Prime Minister". "It's very tentative, making it sound like a remote possibility," said Tom Maddocks, who worked on BBC2's The Money Programme and is course director of Media Training Associates.

Mr Clegg, the trainers agreed, needs to continue to look as if he's not been media trained at all.

Debate Bingo

See how many you spot here tonight:

Risk / Recalibration / Quantitative Easing / Bigot / Greece / Tackle / The other day I met a woman.../ 13 years / Five more years / Debt / Courage / Sacrifice / Black man / Meltdown / Gay / Belize / Ashcroft / Triple-A / I agree with Gillian Duffy / There he goes again / Women – and you are one of them.../ Change / Nutters / Iceland / Is this microphone on? / Regret / Sue / Armageddon / Hard-working families / Vince Cable / The two old parties / Thatcher / Tax on jobs / Get real / Now is not the time for soundbites / Cut(s) / Hung Parliament / Ash / Double-dip / Jeopardise / IMF / Difficult decisions / Bath-time (early) / British public / Whose idea was this?

Statistics Check

£163bn

The annual budget deficit for 2010-11, as forecast by the Chancellor in March

£74bn

The budget deficit by 2014-15, according to the Treasury

£704bn

Total public spending this year

£541bn

Total tax take this year

£44.1bn

Labour cuts to spending as yet unspecified, according to Institute for Fiscal Studies

£34.4bn

Lib Dem cuts to spending as yet unspecified, according to Institute for Fiscal Studies

£52.5bn

Tory cuts to spending as yet unspecified, according to Institute for Fiscal Studies

3.25%

Economic growth for next year forecast by the Treasury

2%

Economic growth for next year according to independent bodies

3.4%

Consumer price inflation (excludes housing costs)

4.4%

Retail price Inflation (includes housing costs)

2%

Official inflation target

£32.5bn

UK overseas trade deficit in 2009

£771.6bn

Current total public sector net debt, colloquially known as the national debt; equivalent to 53.8% of GDP

£1.4 trillion (£1,406bn)

Total public sector net debt by the time it peaks in 2015; 74.9% of GDP.

2,502,000

Total unemployed in the UK, 8% of the workforce

1,543,800

Claiming unemployment benefits

929,000

Youth unemployed (Aged 16 to 24)

35.1%

Unemployment rate among 16 to 18-year-olds

2%

Annual average economic growth under the Conservatives, 1979-1997

2.2%

Annual average growth rate under Labour, 1997-2009