Just an update really. We're basically very pleased with the Jazz after about 6 months.
It's returning between 52 & 54 mpg in mainly local driving on A roads & country lanes - that's actual full-to-full mpg, the onboard display is more generous by 2-3 mpg.
I did the first major journey of about 120 miles each way (M11/M25/M40) earlier this week and it still did more than 50mpg despite maintaining 70mph where possible. OK there is wind-noise at speed but not too bad and it does struggle to maintain speed even on motorway gradients but that's the price you pay for the small engine/long gearing that gives the economy I suppose.
Two minor niggles: I wish there was a positive indication that the AC is ON rather than the tiny message that it is OFF and the windscreen wiper arrangement which always results in a streak from the edge of the shorter one that isn't cleared by the longer one - bit obsessive I suppose!