This is either done directly or via the incident contact centre (RIDDOR) website
Some of these incidents/accidents will be investigated and the outcome of these enquiries usually involves the giving of advice to the employer. If a blatant breach of requirements is identified as the main reason for an accident happening, then formal action will be taken.
Complaints about working conditions and accidents at work are always investigated confidentially. These are usually resolved by providing advice to employers, but formal action will be taken if necessary.
Further information about what is defined as a work-related accident is available on the HSE website