2003 Renault Megane Fuel consumption directly related to ai

Hi, I do the same drive to and from work every day, 15 miles fast country roads at 50 to 60 mph, 17 miles motorway at 60 tp 70 mph, 15 miles cross country at 30 - 40 mph 47 miles in total each way 94 miles per day. Traffic conditions always very similar daily average speed 39mph for whole journey. My megane 1.9 DCI diesel does approx 39mpg if air temp below 0 degrees C, 42 mpg at 0 to 6 degrees, 45 at 6 to 10 degrees, 48mpg at 10 to 14 degrees, 50 mpg at 14 to 20 degrees and 56 mpg at 23 degrees and above. The above MPGs are pretty reproducible at any given air temperature and I get same result if I warm car before resetting computer and have tried holding hose to make sure it stays cold and gets hot suddenly to be sure it is not the thermostats. I think it is an air temp sensor sending duff information to the engine management computer and am told it could be sensor between air filter and turbo - Any ideas and suggestions on how to fix would be greatly appreciated. Thanks Regards David