Excess fuel consumption

[Glovebox]

2012 Mondeo MK 4, 2.0 tdci. I recently travelled over to the UK, with lots of motorway driving. as normal I was getting low to mid 40's mpg. After a long drive up the M1, when I came off the car had developed an erratic tickover, this disappeared the following day. It was then running normally but developed the same problem a few days later,which too cured itself after the car had been parked up all night. Since my return home to France the car is running fine, but now I am only getting mpg in the high 30's. Any ideas anyone, on what the problem might be.