Listed Entities

Hi From my studies I understand there are separate rules for listed companies in comparison to non listed companies. Could you please confirm a few questions I have: - When is an auditor allowed to carry out non-audit work, for example valuation services on a pension plan? (when its immaterial and insignificant?!) - Am I correct in stating that an audit firm cannot prepare financial statements for listed companies unless there is an emergency? Just a few things have confused me slightly and I wanted to get it right! Thank you Jemma