By Nicholas Earl North Sea oil and gas producers are pushing the Government to ensure investment relief is continued in line with any expansion of the windfall tax, warning that the industry is at risk of dwindling without a stable investment climate. Industry sources told City A.M. that North Sea oil and gas firms have been in contact with the Government over the past few days, with senior executives urging the Government to keep the 80 per cent allowance to develop new projects and encourage further exploration in the UK. They argued the investment relief was vital to maintaining confidence …