Just be careful, from what I've seen digging around there is a definite lack of clarity and consistency about the NOx figures between different sites, and there can be a number of figures relevant to a model depending on precise build date etc. The figure should be on the V5 document so I'd be very careful about buying anything without first seeing that number. A bit like with Co2 figures, the onus of proof will be with the person declaring the car. I imagine it will take some time for that confusion to be properly addressed.

As said, EU6 is really a must for any diesel, or EU5 with a great deal of caution. By and large, regardless of EU compliance level, any petrol within the past 10 years at least, will have a lower NOx level than a same year of diesel. Given the graduated pricing structure I see it all but killing importation of any pre-2012 cars (but mostly diesels) and impacting significantly on the mix up to about 2014/15.