There is little in the way of solid evidence to support the use of diesel additives, although some drivers swear by them and others say they make no difference. It's also worth pointing out that no car manufacturers suggest using additives and major hauliers tend not to use them either.

It might be worth trying higher quality diesel fuel to see if this results in improved fuel consumption, but it may just be the fact that as your car ages and the mileage increases a loss of efficiency is inevitable.