Fabia Vrs Fuel Consumption

Can anyone throw any light.

My recently acquired 06 Fabia VRS gave over 60 mpg on a recent 400mile journey. However on the return journey the consumption dropped to high 40s and has remained there. The only possible anomalies noted were on two occasions, when stopped at the roadside looking for directions with the engine running, it seemed to over speed at idle, but then settled. Also I now have noticed that when stationary, but at very low rpm, say 1100, the car / engine seems to shake a little. This clears when 1500 rpm is reached.

The only engine mods, done prior to my ownership was a Jabbasport remap which gives about 170 BHP