The Information Commissioner’s Office has published guidance on what is ‘personal data’ for the purposes of the Data Protection Act 1998. Only data which falls within this definition is subject to the rules imposed by the Act.
The guidance is designed to help data practitioners to decide whether data falls within the definition of personal data where this is not immediately obvious. It uses examples to illustrate circumstances in which data relates to an identifiable living individual.
The guidance can be found at the Information Commissioner's website