David Cameron and Ed Miliband set out contrasting visions yesterday of how to bolster family life and rebuild a sense of community in Britain in the wake of the budget squeeze on public spending.  The Prime Minister announced plans to increase charitable giving and volunteering as he sought to breathe new life into his struggling "Big Society" agenda. The Labour leader, meanwhile, said he wanted to be judged in office by his success in boosting the job opportunities and supply of affordable housing for young adults.  Mr Cameron promised an extra ?42m in help for the voluntary sector in an attempt to encourage people to spend more time and money on good causes. The ambition of fostering a "Big Society" has been a constant theme for the Prime Minister, who has repeatedly returned to it in the face of colleagues' scepticism and public indifference.  Speaking yesterday in Milton Keynes, he insisted that it was fundamental to his determination to improve the quality of life in Britain over the next decade.  "as our debts are paid off, this is what I want to endure as the lasting legacy of this administration - helping to build a society where families and communities are stronger, where our nation's wellbeing is higher, and where all these things are accepted as central, not peripheral aspects of what modern governments should hope to achieve," he said.  "So the Big Society is not some fluffy add-on to more gritty and more important subjects. This is about as gritty and important as it gets: giving everyone the chance to get on in life and making our country a better place to live."  Ministers have been encouraged by the announcement that the Link network