The Health and Social Care Information Centre (HSCIC) publishes data every month (see here for an example) describing the number of drug prescriptions issued by England's GP practices. The data shows the practice level number and cost for every prescribable drug or device (about 30,000 different things at any one time down to different formulations/strengths of the each drug).
Each individual item is described by a BNF (British National Formulary) code that simplicity groups the items into a hierarchical structure (e.g. there is a chapter for "respiratory" with subsections such as "bronchodilators" or "corticosteroids"). There can be four or five layers in that hierarchy. When new drugs are introduced they sometimes require new entries in the hierarchical structure and new BNF codes.
Is there a source for that hierarchy that can be used to keep it up to date with the prescribing data?