There is a well-established formula for an Alan Johnson profile. It is seemingly obligatory to mention his background, orphaned at 12 and raised by his sister in south London and Slough, his marriage at 17 and three children by the age of 20. Then there is the former postman’s trade unionism, guitar strumming, hair gel and sunglasses.

Those traits were a feature of the glowing press coverage Johnson received last autumn, when the education secretary was being seriously discussed as a possible leadership candidate. He has since joined the race to be deputy leader, of course, while endorsing Gordon Brown for the top job.

Having entered a much more crowded field, then, what marks Johnson out from the five other deputy leadership candidates? ‘My experience in the trade union movement, 33 years’ membership of the party, five different constituencies, trade union general secretary, minister for eight years, secretary of state for three different government departments.’ While he praises the other candidates’ experience, he says ‘my kind of mixture, as someone born in London but representing a Hull constituency – I think I can broaden the appeal of the leadership of the party.’

So would he like to see Labour recruit more candidates from working-class backgrounds? He is, after all, the first trade union leader to enter the cabinet since Frank Cousins in 1964. ‘If your point is should we have more working-class people in government, I don’t think you can discriminate in that way,’ he says. ‘But I certainly think you can help people from all backgrounds to positions of prominence in the party. I’ve got Shahid Malik leading up a commission for me, looking at practical steps we can take, including an idea about a Labour college where we can train up people from all backgrounds in positions of leadership in the movement, in the party, at constituency party level, as MPs, as councillors, and as trade union representatives.’

We ask what Johnson’s strategy would be for beating David Cameron. He attempts nothing so coarse as to contrast his own background with the old Etonian, Oxford drinking club, £1,000 dinner jacket Tory leader. Better to highlight Cameron’s ‘inconsistencies’ on policy, he says: ‘The man that in January 2006, after he was elected as party leader, who described a bill that we were taking through at the DTI to give extra maternity leave, extra paternity leave, the ability for mothers to give up some of their maternity leave to their partner, as owing more to political correctness.’

What is more, however genuine Cameron’s newfound centrism, ‘his party isn’t with him’. Johnson suggests that ‘Patrick Mercer [the Tory frontbencher recently dismissed for allegedly racist comments] better reflects the Tory backbenches, and he’s probably on the liberal side.’

Cameron is often said to have charm, however, something the avuncular Johnson clearly has too. How important an attribute is this in a politician? ‘Well, if I’ve got it – yes. If I haven’t – no,’ he charmingly responds. He talks about Tony Blair’s ability to talk and listen to people, and to put people at their ease – ‘Would that I had it in equal amounts.’

We move on to the modern Labour party’s relationship with the unions, about which this former general secretary of the Communication Workers Union has clearly thought deeply. Regardless of the recent creation of the new ‘super-union’, Johnson argues that the block vote wields too much power at party conference.

‘It doesn’t matter whether they merge or not, they can stitch up a vote at conference, irrespective of how the constituencies feel on very important issues, by virtue of the fact they can all vote together and deliver a block vote of 50 per cent. It’s not anti-union to say that’s indefensible.’

But are the leaderships of some unions actually going against the wishes of their members in resisting this kind of change? ‘There are an awful lot of very good, very constructive union leaders out there.’ But, he continues, ‘there is going to be a period now over the next couple of years, in the run-up to the next election, where people are going to have to think very carefully about what they say and how they portray this government.’ This is because the Tories are no longer ‘nonexistent’. ‘Now we’ve got a real fight on our hands,’ he says. ‘That’s the one thing we shouldn’t underestimate about Cameron, he’s quite rightly seeking to put the Tories on the centre ground.’

We close by asking for Johnson’s musical hero, given his well-known fondness for pop music. He has the genuine enthusiast’s inability to restrict his number of nominations, citing Elvis Costello, the Canandian solo artist Ron Sexsmith, Gruff Rhys of the Super Furry Animals – ‘and Paul McCartney, if I can have four.’