Computer only shows the mpg since the last time it was reset, so if the seller reset it, then crawled across the city you could expect a low figure.
My mpg figures are not typical for most Jazz owners. I drive, specifically to achieve good mpg figures. I will go 10 miles to get 50 mpg rather than 2 miles to get 49! It is a game with me. I have a ScanGauge E fitted to my OBD 2 port, and I try to make good use of it. If I am driving on the motorway at 60 mph and I come upon an HGV doing 50 I will tuck in behind him and get the benefit of the tow (not too close as to be unsafe).
I am sure you could get your numbers up just by anticipating traffic and traffic lights (I switch off the engine at known "long" lights). Driving without using the brakes is the best way to learn anticipation. I consider the use of the brakes, other than for stopping and for controlling my speed on downhill sections, to be a failure on my part. There is no point in accelerating then braking, both using up needless energy (fuel), when keeping a constant speed would achieve the same end but with no wasted energy. I try to avoid having the car come to a complete stop at lights and in queues, as the biggest waste of energy is getting moving again.
Some good tips to be found here
https://ecomodder.com/forum/EM-hypermiling-driving-tips-ecodriving.phpbut no need to go to some of their extremes.