Many people use a can of Forte fuel treatment just before the MOT, well regarded products.
I and several others on this site use Millers Diesel treatment every fill.
Some on this site call such products snake oil, you pays your money and takes your chances.
Hope the car in question gets regular oil changes, and if the air filter has been left for ages that suggests a new fuel filter might pay dividends, could the worsening fuel consumption be more brake related one wonders, again bearing in mind the air filter.
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