I have no experience of CMax diesels per se, as I also drive a 1.6 petrol, but I do know someone who drives a 2.0 Avensis diesel. He sometimes gets over 50mpg on a long run, but only 25mpg around town and about 35mpg on a short commute, like the one you do. In my Cmax I get 33mpg around town, and about 40mpg on a long run, and, like you, somewhere in between on average. Given that I do only 5,000 miles a year, and most of that is flitting about, I reason that I am perhaps a little worse off in terms of mpg, using a petrol engine, but given the kind of driving I do, the difference is very marginal. If I drove a big mileage and took a lot of long journeys then, no doubt, a diesel would be a much better option. Methinks that if you want 40+ mpg guaranteed whilst doing any kind of commuting then you should be looking for a much smaller car.
The other points of consideration simply echo those made above. The impression, at least from this site, is that the petrol engines are more reliable. I may be wrong about this, given that diesel owners far outweigh petrol owners hereabouts, but I, at least, have avoided any 'out of hours' expenditure on my Cmax.
Edited by: romseyraver