I brought my car at 5000 miles on clock this summer and got a steady 51 mpg. I am now onto 9500 miles on the clock and for the past 2 weeks am getting 48 mpg. Same journey, same style of driving. Why this sudden change? Is it the cold weather. My heating is not on yet in the car. Any guesss?

51mpg to 48mpg isn't a great change. Yes the weather is probably to blame. The engine will take a little longer to warm up. Things like tyre pressures can have an effect.

It will get worse as the weather cools off. The cooler air is denser and harder to push through, lubricant in the axles and drive train is also stiff until you've driven for a bit.

Heat being on should not have a noticeable effect on MPGs, its not like air conditioning.

All down to how quick the engine gets to operating temperature. Will take longer now autumn/winter arriving, im noticing the same , and as previous post cold gear oils also will have a slight effect, lights on, wipers , electrical loads also

I just wish my 125 ecoboost was anything as good as yours. When I got mine 2 years ago the mpg was in the low 30s, now with 22000 on the clock I'm lucky to get 36. A recent long journey produced approx 42 mpg