Came across an article on air quality on the Grauniad which referenced Luftdaten.info and a Friends of the Earth testing kit (donate £15-£50 and receive an NO2 kit, "including a full specialist lab analysis of your air monitoring tube").

This might help with the calibration. Luftdaten.info has a standard setup they recommend (including a simple idea for weather proofing the sensor array), and they are a national project run by a German university. Encouraging to see they use the PPD42-type sensor. And FotE will perform a lab test on an NO2 sample from the site - although it costs minimum £15 and the test runs for 2 weeks.