They sell it in lots of places, eg Tesco's, Asda and poundland all carry it. Comes in different types though I'm not sure of the differences but it's basically an additive for fuel, Personally I have had great success with it, both the diesel formulation which I used in my 4x4 dumper that was getting cranky to start - not good news on a hand cranked diesel but pre dosing it with redex you could crank all day if the temp was under 10°c and if wouldn't start, after a dose it now starts first try right down to freezing.