On further reading discovered that Luftdaten.info are now recommending the (more expensive: c. £20) SDS011 particulate sensor. Need to investigate for the rationale, and to discover whether they are better than/as good as the laser-based PMS 7003.
This evaluation of the PPD42NS at PLOS ONE. Note the concern over accuracy at high levels of particulate concentration.
This description of a PPD42NS-based air quality monitoring kit at howmuchsnow.com.
This blog post about calibrating the PPD42. In the comments (May 15, 2016 at 7:37:20 pm) A.J. recommends the SDS011 as being laser-based - which matches the Luftdaten.info change.