Employers are under a duty to take reasonable care for the safety of the people who work for them. That duty is not vague: it is supported by health and safety legislation that sets out specific obligations, and by regulations dealing with particular risks such as working at height, manual handling, machinery and the provision of protective equipment.
The practical consequence is that a workplace accident is rarely an unlucky one-off. Where something has gone wrong, there is usually a step that was missed: a risk that was not assessed, equipment that was not maintained, training that was not given, a job that was rushed to meet a deadline. Identifying that missed step is what establishes responsibility.
It also means there is usually documentary evidence. Employers must keep records of risk assessments, training, equipment inspections and maintenance, and incidents. Serious and specified accidents must be reported to the Health and Safety Executive or the local authority under RIDDOR, and an enforcement investigation can produce material that is very useful in a claim. We request these records as a matter of course.