Plagiarism is not permitted. This refers both to re-using any material including your own without appropriate attribution / referencing and to re-using previsouly assessed material in later assessments. For example, you may not pull material directly out of your Master's dissertation and re-use this in your PhD thesis. There are nuances to this as with anything - you may re-use short sections of material if it is properly acknowledged. 'Short sections' may be a matter of opinion and depend as much on the nature of the work itself as on the actual length of the excerpt.
Students should refer to the University Calendar for more information, in particular the section on 'University Fees and General Information for Students' which contains the University's Plagiarism Statement. Specifically:
Plagiarism is defined as the submission or presentation of work, in any form, which is not one's own, without acknowledgement of the sources. Plagiarism includes inappropriate collaboration with others. Special cases of plagiarism can arise from a student using his or her own previous work (termed auto-plagiarism or self-plagiarism). Self-plagiarism includes using work that has already been submitted for assessment at this University or for any other academic award.