Any car around town, whether diesel or petrol, will be dire on MPG. I average 30-32 over a tank which is a mix of going through town and on A roads (not dual carriage / motorway). On the motorway I get 35-37 on the 18" alloys and 37-41 on the 16"s.
It also depends of course on how you drive, looking ahead so not braking unnecessarily, tyre make & condition, general engine condition, petrol type etc.
If you are worried about MPG, you have the wrong car, but it is something people seem obsessive with more recently when in most cases over the period of a year the cost isn't that much of an issue (compared to servicing, tax, insurance, repairs) unless doing a lot of miles. I'd rather pay a few hundred quid more in fuel than drive some dag dag.
