By Nicholas Earl The UK risks no new investment in the North Sea oil and gas sector, if the Government toughens up the windfall tax at the fiscal event this week, warned investment analyst group Stifel. In his latest report on the oil and gas sector, managing director Chris Wheaton predicts that future investment will only consist of pledges already made by energy companies for future investment in British waters. He told City A.M.: “The combination of higher tax and uncertainty about the tax environment, rather than solely the tax itself, is what is likely to curtail investment, as companies …