WHEN the Liberal Democrats originally proposed a “mansion tax” in 2009, it hit a nerve with the electorate. The party has since reiterated its pledge to get owners of homes worth over £2m, mostly in London’s frothy housing market, to pay a levy. The Labour Party has followed suit; Jim Murphy, its leader in Scotland, this month defended the policy stating it could pay for 1,000 new nursing posts there. But Peter Mandelson, a Labour peer and former minister, this week lambasted the proposal as “crude” and “short-termist”. Critics say the tax is just as likely to catch cash-poor, asset-rich people who happen to have lived in the capital for a long time as it is the genuinely rich. According to analysis by Knight Frank, a London estate agent, 2.5% of all residential properties in Greater London would breach the £2m threshold. Two-fifths of these properties are flats and roughly the same proportion again are terraced houses. If the same top 2.5% calculation were applied to prices outside London, the point at which a house becomes a "mansion" would vary enormously around the country. In the North East, for example, a mansion would cost £344,000, just 25% higher than the price of the average home nationally.