This news article is worth a read for A level Economics students relating to externalities and market failure.  See below exactly where to find the relevant part of the specification for Edexcel, AQA & OCR:
Edexcel spec reference 1.3.2 Market Failure
AQA spec reference Positive and negative externalities in consumption and production
OCR spec reference Page 14 Externalities
APT have A Level Economics resources available for all 3 boards –