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It could be the coolant temperature sensor giving a false reading. IE showing engine temp lower than actual to the injection computer. It could be the thermostat, does the temp gauge read like it's always read?
It's also been quite cold recently, but I only loose about 5mpg in the winter months.
Change your air filter and pump up your tyres to recomended. This was happening to mine. Airfilter was really dirty. MPG is now back up to 42. and hopefully in summer 45