I've been going through this myself. One of the problems is that most of the advice work out that you keep a car 3 years, which isn't very long in my opinion.
I have kept my car 10 years and am now considering changing. It's a fast(ish), 2 litre petrol and while I get low 30s if I drive it hard, I can get over 40mpg on mixed driving and 45+mpg on motorway driving. Servicing is 100pound a year generally but it's just had a major for 180. It's a Primera so a similar size to the Passat mentioned above which is getting 50mpg and is being branded as great economy. In comparison, it's looking good value for money and it's a big car which is where a diesel should be better. Economy and performance drops if it's laden up and I guess this is where a diesel would excel.
I quite fancy a Honda Jazz also and would probably get a petrol.