Rare boost for Osborne as tax raid on multi-million pound mansions raises 50% MORE than he expected Property experts say increasing stamp duty on homes worth more than £2million to 7% has raised £223million Treasury had expected to rake in around £150million in the first David Cameron to give big speech on economy on Thursday in bid to calm Tory jitters, Downing Street reveals Aides to George Osborne urge panicked Tories to 'calm down' By Matt Chorley and Jason Groves George Osborne introduced the stamp duty increase for homes worth more than £2million in last year's Budget Multi-millionaires buying and selling some of the most exclusive homes in Britain have paid out almost 50 per cent more in extra tax than George Osborne had expected. The levy, introduced by the Chancellor last year amid calls from the Lib Dems for a full-scale mansion tax, was predicted to raise around £150million. But while the rest of the economy has remained in the doldrums, house prices for the most luxurioes properties in London have continued to soar. As a result, property experts expect HM Revenue and Customs to rake in more than £223million by next month. The extra £73 million will provide a welcome boost for Mr Osborne, who is under growing pressure to unveil eye-catching tax cuts in the Budget on March 20 to get the economy moving. Growing Tory demands for action to tackle the rising cost of living have intensified since the party suffered the embarrassment of coming third last week's Eastleigh by-election. Today it emerged David Cameron will deliver a major speech on the economy on Thursday in a bid to calm Tory jitters. The Prime Minister will use the event, at an undisclosed venue in the north of England, to drive home the Prime Minister's message on the need to stick to the course on economic policy. It comes amid increased squabbling within the coalition Government over spending plans, with Defence Secretary Philip Hammond going public with a call for welfare to bear a greater share of austerity pain. Mr Osborne hiked the stamp duty levy on properties worth more than £2million - most of which are in London - to five per cent. Property consultancy Knight Frank said that in the aftermath of the announcement in last year's Budget the number of homes sold for more than £2million fell by 23-35 per cent. But they have picked up since, and those that have sold have raised even more for the taxman. Liam Bailey, head of Residential Research, said: "Knight Frank have calculated that by the end of March, HMRC will have collected some £223m in additional tax revenue since the 40% rise in stamp duty was implemented in 2012, £73m more than originally forecast." The Treasury targeted sales of high value properties, like these flats in Knightsbridge, but have been surprised by it raising more than expected News of a £70million tax windfall for the Treasury news comes as Mr Osborne faces growing calls to "change direction" in this month's Budget - amid mounting alarm about both the economy and "incoherence" at the top of the party. As demands for an eye-catching tax cut build a head of steam, Mr Osborne's parliamentary aide Amber Rudd urged panicked colleagues to 'calm down'. She told BBC Radio 4: 'We can't go round giving unfunded tax cuts. That is absolutely the Government's position. I think it would be very unlikely that they will get that, because we've got to have a very prudent government that's going to restore financial stability where it's been so absent for the last 13 years before the general election, and we've got to make sure our cost of borrowing is kept down.' UKIP leader Nigel Farage said voters no longer believed Tory promises.