DAVID Cameron has promoted the Conservatives as the party for working people with plans for a tax-free minimum wage across the UK, doubling free childcare and extending the right to buy programme south of the border as he warned against the "horror" of a Labour-SNP alliance.

Launching his party's manifesto at a technical college in Swindon, Wiltshire, a Labour-Tory marginal, the Prime Minister declared: "At the heart of this manifesto is a simple proposition: we are the party of working people, offering you security at every stage of your life."

In an address, that was upbeat and positive - in contrast to what critics have said has been a deeply negative campaign thus far - Mr Cameron said a Conservative government would use the next five years to turn "the good news in the economy into a good life for your family".

The policy programme contained already known proposals such as lifting the inheritance tax threshold on family homes to £1m by 2017, no above-inflation rises in rail fares until 2020, an extra £8bn a year for the NHS in England by 2020 and an in/out EU referendum by 2017.

But there were three main policy announcements, all designed to expand the Tories' appeal from its traditional base.

There was a boost for the 1.4m people, 150,000 in Scotland, on the minimum wage and who work 30 hours a week. By 2020, the Conservatives intend to raise the tax-free personal allowance from the current £10,600 to £12,500, which will ensure no one on the minimum wage pays tax.

But Mr Cameron went further and announced the Tories planned to change the law to ensure the personal allowance kept rising in line with the minimum wage.

"If Conservatives are in government, we will change the law so that no one earning the minimum wage will pay income tax. Yes: the tax-free minimum wage," declared the Tory leader, insisting this was a sign of modern, compassionate Conservatism.

"This is a landmark change. It means that we can proudly say that this is the party of working people," the Tory leader said to applause.

Party sources brushed aside suggestions that this could in future cost taxpayers dearly if the Low Pay Commission recommended the level of the minimum wage should be raised significantly. A senior source said the proposal to legislate on automatically uprating the tax-free allowance to keep in line with the minimum wage would "lock in" future governments. "If they don't keep it, then this will be seen as a tax rise for the low paid," he claimed.

On childcare, the manifesto outlined the 15 hours a week of free childcare legislated for in the last UK Parliament would be uprated.

"Because working families with children under school age face particularly high childcare costs, in the next parliament we will give families where all parents are working an entitlement to 30 hours of free childcare for their three and four-year-olds."

This will save parents £5000 a year but will only apply to England as childcare is devolved in Scotland. But the 30 hours is the same level as the Scottish Government said it wanted in its White Paper on independence and what First Minister Nicola Sturgeon earlier this year said her party had set its sights on achieving by the end of the decade.

The childcare proposal will cost £350m once tax credit savings have been taken into account and will be paid for from changes to top earners' pensions. It will affect 630,000 children and come into effect from 2017.

Mr Cameron said: "A good life should mean that raising your family feels like an incredible and joyful and, yes, sometimes exhausting journey but it shouldn't be a struggle with the bills."

The third major announcement was on housing with the party leader setting out the proposal to extend the right to buy council houses in England, introduced by Margaret Thatcher in the 1980s, to housing association homes. This could affect up to 1.3m tenants.

In Scotland recently, the Scottish Government announced its plan to scrap the right to buy because of massive housing waiting lists, so the Tory plan to extend it to housing associations would not apply north of the border.

While there was no mention of the SNP in the PM's speech, in a subsequent Q&A session he issued yet another warning about the prospect of Labour forming a pact with the Nationalists.

"Here is the point of this election. If you want to stop the horror of a Miliband government backed by the Scottish National Party, it's no good voting Liberal Democrat, Green or Ukip, you have to vote for the Conservative Party as the only party that will secure a majority government to keep Britain on the right track."

Insisting Labour would put the recovery at risk, Mr Cameron stressed: "I don't think they have learnt anything from the last five years and their manifesto gives them the leeway to do it all over again; they won't take any prompts from Nicola Sturgeon or Alex Salmond to make them do it all over again but, believe me, if there were some, it would be even worse - even more borrowing, even more spending, even more out of control welfare. That's the choice."

But Ed Miliband dismissed the Conservatives' manifesto, accusing Mr Cameron and his colleagues of having "absolutely no idea" how this and other spending commitments would be funded.

"The reality about the Conservatives is that they are the party not of working people; from first to last and always, they are the party of the richest in our society and that is absolutely the case with what they are saying today," he said.

Meantime, the Liberal Democrats branded the Tory plans a "total con" because there were no details of future cuts to public spending and welfare.

"Until they answer the question of who pays, people will just see this manifesto as a combination of secret cuts and unfunded spending commitments," insisted Danny Alexander, the Chief Secretary to the Treasury.