I found out by chance that an emissions compliance certificate is required for diesels in France - a few Euros. I am trying to find out what other countries have similar rules. Germany seems to have a system but I cannot tell if it is a scam or not - they want 17 Euros for the same thing!
I am planning on going over in June and not returning until September, covering France, Portugal, Spain, Netherlands, Belgium, Poland, Latvia, Lithuania, Estonia, Switzerland and Italy.
It seems to be impossible to find a centralised site with any coherent information on it.
If anyone knows of one, or has knowledge of the requirements in any of those countries, please let me know.