I live in rural France too, and also only use supermarket diesel fuel and have done for the past 8 years, vehicles include a 2005 Mitsubishi L200 diesel pick up, a 2007 Honda Civic 2.2 diesel and currently a Honda Accord Type S 2.2 diesel.

I am of the opinion that French diesel fuel is better quality than the UK stuff - certainly gives better mileage - I have never had problems with any of the aforementioned vehicles and have never used additives as such.

However I do add 200 ml of synthetic 2 stroke oil to a full 70 litre tank of diesel on the pick-up, to give the old-fashioned rotary distributor injection-pump an easier life-no other reason.

Informed advice seems to say not to do this on a modern common rail engine like your mondeo.