Interestingly enough, they were always there, just not worried about. Brown persuaded everyone to go diesel because it helped slow down global warming , but we're now being persecuted for something they helped cause. But modern diesels burn off the NOX with the cat converters (or their equivalent) and have diesel particulate filters to cut down the smog effect, so why are Westminster council just going to target diesel drivers en masse, instead of just older ones.
When this problem started to raise it's ugly head, I read an article that said that petrol engines were killing as many people as diesels were, just with a different set of respiratory diseases.