Visa extention due to dissertation failure

"If your institution is willing to issue a CAS, you can apply as a Tier 4 (General) student to re-sit an exam or to repeat a module as part of a course that you are already studying.

You may only be allowed to be in the UK as a Tier 4 (General) student for the length of time that you need to re-sit the exam or repeat the module. You will then have to apply for further permission to stay in the UK and complete your course.

You will be limited in the number of times that you can apply for immigration permission to enter, or stay in the UK as a Tier 4 (General) student in order to re-sit an exam or repeat a module. This will depend on whether your institution has highly trusted sponsor status.

Many institutions have decided to not issue a CAS for an examination re-sit. They will expect you to leave the UK once you are no longer required to attend your course, then return as a student visitor for the re-sit. Having re-sat your exams as a student visitor, if you will need more time to complete your course, you must return home and apply for entry clearance as a Tier 4 (General) student"