The whole thing about diesels being worse is rubbish.
When the government started their anti diesel campaign many many scientists and experts spouted up and totally dispelled the data they where basing it on.
Even in the US they have large diesel pickup trucks with dual cats producing little more than water vapour.
The figures go awry when they base pollution levels on quantities produced per litre of fuel burned.
This process is biased towards petrol as diesels tend to to travel a lot further on that litre of fuel thus use less. It should be pollution per mile travelled for an accurate result.