As the employer if there is an accident connected with your work and an employee, (including self employed / contractor) or a member of the public suffers death or a major injury then you are obliged to notify the HSE incident contact centre (0845 3009923) immediately by phone or on line (via HSE website).
A reportable “over 3 day injury” even includes injury caused by physical violence.
RIDDOR also applies to ‘Reportable Diseases’. If you are notified by a doctor that your employee is suffering from a reportable work related disease, e.g. occupational dermatitis, skin cancer, then notification must be made.