Thanks for everyone's replies.
Since my original post, I have refuelled the tank and also done a service on the car, consisting of Honda Hybrid Oil and a new Honda filter. The temperature is also slightly better, I recall it was around 5 to 10 degrees every time I got into the car.
What puzzles me is the MPG I read some Toyota Yaris Hybrid owners are getting, up to 70 MPG!
Form my limited research, it seems to me the differences are (happy to be corrected of course!) as follows........
The Jazz Hybrid came out in 2011 (1.3 litre), the Yaris Hybrid in 2012 (1.5 litre). Unless you are standing still or moving at walking pace, the engine on the Jazz does not switch off and the electric motor only assists. However, in contrast, the Toyota’s Hybrid system can drive on (a) electricity only (b) engine only (c) both together at the same time.
I have read that up to 500 miles on 30 litres is possible.
Honda came out with the original Insight in 1999 (they were the first I think), they have (or had?) an F1 team, they have built some incredible cars (Honda NSX, for example) and spend untold amounts on R&D. I'm surprised the Toyota can better it in the MPG stakes.
Oh another thing - I test drove a Yaris Hybrid - I only drove a poverty spec version and I have an HX so hardly a fair comparison, but overall the interior in the Jazz is far nicer.