Environment Agency H4 Odour Guidance - The document that details numerical odour standards for odour assessment.
2002 Draft of H4 - This was a much more extensive document and has some good background information.
Defra Local Authority Odour Guidance - useful guidance covering many aspects of odour assessment - note that Defra withdrew this guidance in September 2017 but no replacement has been issued.
IAQM Guidance on Odour and Planning - the only guidance relevant to planning - please see IAQM website.
Defra Guidance for Commercial Kitchens - contains the methodology used for a risk assessment of odours from restaurants/takeways - note that Defra withdrew this guidance in September 2017 but no replacement has been issued. An unofficial update was produced by EMAQ+.