Data released by the UK Treasury last week showed that the tax take from oil and gas collapsed from £8.8bn in 2010-2011 to £4.7bn last year, the lowest share of GDP for a generation. The fall in revenues would make it even harder for Scotland’s Alex Salmond to cover his spending plans with North Sea revenues if the Scottish Nationalists win the referendum. The revenue slump occurred even though Brent crude prices have been above $100 a barrel, buoyed by supply disruptions in Iraq, Libya and parts of Africa.