http://www.southsomerset.gov.uk
An somewhat large but good website which includes an impressive compilation of guidance and information on Environment and Building Conservation, though you may need careful digging / selection of 'search keywords' to find it. For example, a search for the phrase 'mixes and proportions' leads you to http://www.southsomerset.gov.uk/agenda21/buildingcons/lime book/mixesand.htm, which lists criteria for recommended replacement lime mortar and render mixes versus various applications and exposure situations.
Alternatively, you can use the Site Navigation ('Go To') bar at the top of the website to go to the 'Conservation' and 'Building Conservation' areas of the website, or any other area of interest.
The Section 'A Guide to the use of Lime in Historic Buildings' is recommended.