when there is existing law already created by parliament, the judges job is to apply and interpret the law to the case that they are hearing. this is called statutory interpretation. it is down to the presiding judge how they interpret the law made by parliament to the case they are hearing (although their are some guidelines). in reality judges should not be making their own laws as this goes beyond their powers as judicial decision makers. however, where there is no law make by Parliament, sometimes judges do have to create law as they go along. this is known as judicial precedent.