By Kylie MacLellan and Paul Taylor BRUSSELS (Reuters) - David Cameron struggled at a European Union summit on Friday to overcome last pockets of resistance to a deal designed to keep Britain in the 28-nation bloc, with diplomats forecasting an agreement within hours. Cameron was hoping to fly home and chair a cabinet meeting later on Friday to endorse what he calls a "new settlement" with the EU, setting in motion plans to call a referendum on Britain's future in the Union, probably for June 23. The stakes are high for both Britain and the EU, with opinion polls showing voters almost evenly split.