Euro 5 regulations for diesel cars were staggered, which is why there is a Euro 5a and Euro 5b. Euro 5a was introduced in 2009 and Euro 5b in 2011, so given that your car was registered in 2015 it should be Euro 5b.

However, we would always recommend checking the government website before driving into any low emission zones, which you can do here: www.gov.uk/clean-air-zones