ARMA Publications
RICS
Leasehold Legislation
The Acts that regulate the leasehold sector are available in The National Archives with easy access links to all the sections, and a search facility to find them. Here are links to two key Acts:
Energy Efficiency and Insulation in Blocks
Wall Insulation
National Insulation Association leaflets on how to insulate houses (they’re working on a leaflet specifically for flats, but these house ones are still instructive).