Andrew Grice: Big Society? Big silence. And that's just one of Cameron's new problems

"Forget the child benefit cuts. There's something else that will blow your socks off," the senior Conservative said.

He was talking about George Osborne's announcement on breakfast-time TV that child benefit would be axed for families with someone on the 40 per cent tax rate, and the speech he would make to the Tory conference five hours later.

The "something else" was the Chancellor's announcement of a £26,000-a-year ceiling on benefit claims by one family, in line with the median income for working families. The Tory high command seems to have calculated that another crackdown on claimants accused of milking the system would trump the child benefit decision and win widespread approval in Tory-supporting newspapers, which love stories about social security scroungers. It was wrong.

The child benefit axe will affect 1.2 million families, the benefits ceiling only about 50,000. The Tory papers saw the child benefit cut as a much bigger story, an Exocet aimed at their readers. Instead of blowing our socks off, jittery Tory MPs wondered whether Mr Osborne had shot himself in the foot.

The jitters spread. The following morning, David Cameron appeared to wobble under fire when he took to the breakfast-time shows. He wheeled out an old favourite to steady Tory nerves and placate stay-at-home mums angry about the child benefit cut – transferable tax allowances for married couples.

Mr Cameron, a follower of the New Labour playbook in many respects, had previously declined to copy Tony Blair's strategy of taking on his own party. He repeatedly said he would not "pick a fight" with Tory traditionalists. But times change, and, to show that everyone has to make sacrifices, the Tory leadership judged that it needed a symbolic cut to hit the party's natural supporters. Announcing it at the Tory conference, where the audience would be among the victims, would maximise the impact. "We needed some self-inflicted pain," one senior figure said. That would give Mr Osborne the space to restrict benefits to those at the bottom end, showing we really would be all in this together.

As a by-product, the Chancellor also saw a clever trap for Ed Miliband. He calculated the Labour leader would either have to accept a breach in the principle of universal benefits or explain how he would find £1bn to protect handouts to the relatively well-off while its core supporters further down the income scale were hit by other cuts. But when it was his turn to do breakfast TV on Thursday, Mr Miliband was happy to oppose the cut. If the Tories are going to risk a founding principle – locking the middle classes into the welfare state so they get something back for the taxes which help those at the bottom – Labour is not ready to join them.

By the end of the Tory conference, some of the party's MPs were wondering whether the leadership risked surrendering a weapon that could have been fired at Mr Miliband. When he, rather than his brother, was elected Labour leader, Tory strategists believed they would be able to portray him as "anti-aspiration" because his campaign pitch was for Labour to woo disenchanted working-class voters rather than the middle classes. The danger for the Government is that the child benefit cut, which takes effect when one earner is on £43,875 a year, could also alienate people just below the top rate threshold who aspire to move up the ladder.

The Tories already have problems with the aspirational middle classes who flocked to Margaret Thatcher and later to Mr Blair. There was widespread agreement at a ConservativeHome fringe meeting in Birmingham when disaffection among this group was cited as one reason why the Tories failed to win an overall majority in May.

That failure was the elephant in the conference room. No one wanted to talk about it. "We have moved on," one cabinet minister told me with a smile. Some Tories are unhappy about the lack of a proper inquest. "The Cameroons won't engage, but we should be learning lessons," said a key player in the Tory campaign.

Cameron allies say there would be little to gain from a public post mortem. Now the party is in power, they say, the next election will be very different to the last, and the coalition will make it more so.

The Cameroons insist their man does not regard people on £44,000 a year as rich. Yet some Tory MPs worry that the Cameron-Osborne team might give that impression, reviving latent suspicions among voters that they are out of touch because of their privileged backgrounds. Alan Johnson, the former postie and the new shadow Chancellor, will not wage class war against Mr Osborne, but his life story makes an interesting contrast to the Chancellor's.

In his conference speech, Mr Cameron gave his most passionate explanation of his "big society" vision. His challenge now is to sell it to the public at a time when it might be seen as cover for the huge spending cuts to be announced by Mr Osborne in 11 days. At the ConHome fringe meeting, the "big society" was blamed for a lack of coherence in the party's election message. The activists in the audience were asked whether they had found it a useful campaigning tool. No one put their hand up.