It was a measure of the complexity of medical negligence litigation that Lord Woolf in his now famous and influential report devoted a special mention to those proceedings.
A plaintiff must prove the liability of the defendant. This is not equivalent to proving causation. Liability may arise where proof of an error in judgment or management is established, but the plaintiff must go in to prove that that error was the cause, or a cause, of the untoward outcome for the patient.
Very often this requires the evidence of two separate experts on behalf of the plaintiff, one on liability and the other on causation.