My mpg is all over the place. Some days i can get 47 mpg on a long journey (last week) today only had about 38. Seems to be weather based, dry run is a lot better than wet days. Did the same last month, 46 up to glasgow in the dry, 41 back the next day in the wet. Same roads same speeds. Have just had it serviced and that didn't throw up any issues. What is going onnnnnn?

Weather can have a noticeable affect on fuel consumption. I do the same 600Km round trip (mostly highway on cruise control) regularly in my TDI and have noticed the following:

Cold weather (below 15c) increases consumption (and power).

Cold starts increase consumption. I notice a big hit when starting from cold, especially in cold weather.

Warm weather (20-25C) decreases consumption.

Hot weather (above 29C) increases consumption (aircon ).

Wet roads increases consumption (I think the tyres use energy to pump the water out of the tread, I was expecting reduced consumption due to lower road friction, but it doesn't work out that way).

Head and side winds increase consumption.

Tailwins reduce consumption.

Higher cruising speeds increase consumption.

Windy/hilly roads increase consumption.

Traffic holdups increase consumption.

Running with headlights on doesn't seem to make much difference (except to how long your globes last for).