Don't ever be tempted to use supermarket petrol in your Jazz!
My wife used to use it and her Jazz always felt sluggish compared with mine, I got her into the habit of using 'normal' fuel and not only is it a lot livelier, but does more mpg so more than makes up for the 5p per gallon off offered by Tesco, Sainsbury's etc. (and no danger of silicon in the fuel wrecking your Lambda sensor).
Shell 'V' power or BP Ultimate (97 RON) get even better mpg, but getting pricey now