The HSE (Health and Safety Executive) says that businesses make a report under RIDDOR (The Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013) when: 

• an unintended incident at work has led to someone’s possible or actual exposure to coronavirus. This must be reported as a dangerous occurrence.
• a worker has been diagnosed as having COVID 19 and there is reasonable evidence that it was caused by exposure at work. This must be reported as a case of disease.
• a worker dies as a result of occupational exposure to coronavirus.

Read more about what you need to report and how here.