"This week, Professor Frank Kelly, chairman of the Department of Health's committee on air pollution, said diesel engines could be responsible for more than 7,000 deaths a year because of the pollutants they emit."
...In comparison to what? To 1,000 deaths caused by Petrol, or to 10,000? The paragraph above is meaningless without a comparative figure.
That fumes from burning fosil fuels kill people, we already know. The question was however if switching from Petrol to Diesel increases or decreases the number of deaths per year, and this is not really answered by the Daily Mail.