Thanks for that. It is the 1.5 diesel.

Another factor is that winter and summer diesel have different formulations to prevent "waxing". The winter formulation may have a lower calorific value so needing more to be burnt for the same power.

According to wikipedia

"The United Kingdom differentiates diesel fuel into Summer and Winter Fuel according to BS EN 590 and BS EN14214 (biodiesel)..... Distribution of winter winter biodiesel in the United Kingdom starts in November and ends in March".

Designation CFPP value time frame Cloud Point
Summer Fuel -5 °C 16.03. - 15.11. 3 °C
Winter Fuel -15 °C 16.11. - 15.03. -5 °C

Thanks again for your input. Some interesting comments. As previously said, I always check mpg each fill, but as this has only been a recent operation, ie using the AC permanantly, it'll be interesting to see the results when in the winter months..

I don't monitor my MPG that closely, so I couldn't say whether our Jazz is more economical in winter than summer. However, it never seems to drop below 50mpg according the dashboard display, and that'll do for me.

The explanation of the difference between winter and summer consumption seems obvious, with, as stated above, the engine running at higher revs for longer in winter before it reaches operating temperature.

I read somewhere recently (might have been on here) that AC should be left on permanently during the summer, employing a "use it or lose it" ethos. No idea whether that's true, but I only use mine when I need it, which is currently "most of the time".