I work out my mpg anally every fill up. 
Between 50-55mpg in any 306 HDi, fully loaded estate, standard estate with broken MAF, heavy stage 1 map hatch, shit stage 1 hatch, ect etc, doesn't matter what you do to them, I cant get better or worse lol!
For comparison to give an idea of my driving style - averaged 60mpg in a mapped PD Golf, 55mpg in a standard PD130 Golf, 34mpg in a 1.8 306, 48mpg in a Vectra CDTi150, 45mpg in a 1.4 306.
Between 50-55mpg in any 306 HDi, fully loaded estate, standard estate with broken MAF, heavy stage 1 map hatch, shit stage 1 hatch, ect etc, doesn't matter what you do to them, I cant get better or worse lol!
For comparison to give an idea of my driving style - averaged 60mpg in a mapped PD Golf, 55mpg in a standard PD130 Golf, 34mpg in a 1.8 306, 48mpg in a Vectra CDTi150, 45mpg in a 1.4 306.