One-stop source of information on Low Emission Zones (LEZ) in Europe, run by Sadler Consultants on behalf of a network of cities and ministries that operate LEZs www.lowemissionzones.eu. 

The (London) Mayor's Air Quality Strategy*: www.london.gov.uk/mayor/environment/air_quality/index.jsp
In particular it includes:
Measures to clean London's air (Chapters 4 & 5)
Guidance for the 33 London boroughs (Sections 5B & 5D)
Context and air quality situation as of 2002 (Chapter 3)
Legal basis of the air quality management system in the UK (Chapter 1).

The London Low Emission Zone feasibility study*:
www.tfl.gov.uk/roadusers/lez/default.aspx

Reports on the Central London Congestion Charge*, the monitoring reports are at the end of the list on this page: www.tfl.gov.uk/tfl/cclondon/cc_publications-library.shtml

The London Construction Best Practice Guide, including cleaner vehicle requirements: www.london.gov.uk/mayor/environment/air_quality/construction-dust.jsp

UK Government air quality monitoring website, with UK air quality information, Government air quality reports, guidance, and in fact a Government repository for air quality information: www.airquality.co.uk

UK national atmospheric emissions inventory: www.naei.org.uk

UK Government Guidances, helplines, best practice - very useful site:  www.uwe.ac.uk/aqm/review/index.html

London Air Quality Monitoring Data: www.londonair.org.uk

EU Cafe Programme, the EU review of air quality: http://europa.eu.int/comm/environment/air/cafe

National Society for Clean Air (NSCA): www.nsca.org.uk

More about Clear Zones, improving town centres and reduce traffic and pollution: www.clearzones.org.uk

UK Environment Agency, regulates the larger industrial processes in the UK: www.environment-agency.gov.uk

UK Government (DEFRA)'s Air Quality Page: www.defra.gov.uk/environment/airquality

* Denotes work Lucy Sadler, from Sadler Consultants played a key role in, during her previous role as head of air quality for the Mayor of London, at the Greater London Authority.

Sadler Consultants takes no responsibility for the content of external websites.