You must only 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.

More information regarding RIDDOR:

https://www.hse.gov.uk/news/riddor-reporting-coronavirus.htm?utm_source=govdelivery&utm_medium=email&utm_campaign=coronavirus&utm_term=riddor-1&utm_content=digest-8-apr-20