UK gas bills have soared by more than 50% over the past five years despite British households using 17% less gas, new research has found.The research, carried out by think tank Centre for Economics and Business Research (CEBR), found households now use 16,000 kilowatt hours per year, compared to 20,500 kilowatts used five years ago.A switch to more efficient condenser boilers and better insulation caused the fall in gas usage, the think-tank said. "Using detailed modelling techniques we were able to determine that energy efficiency measures are the primary driver of the average annual decline in underlying gas consumption," Jonathan Thurwell, director of competition economics at CEBR, said.But this has not meant smaller bills for customers. In 2006 the average household gas bill was £447 compared to £657 now. Over the last eight years, bills have more than doubled.In December, energy regulator Ofgem launched an investigation into energy companies after fresh data revealed a sharp increase in profit margins.