If it is clear that you will not be ready to submit by the usual deadline (the end of your tenth term for full-time PhD students), you can apply to defer submission of your thesis. Applications are administered through CamSIS, the Cambridge Student Information Service:
You must give the reasons for delayed submission on the form and estimate, realistically, how much longer you will need. Approval of applications is not automatic, especially for deferral beyond a fourth year, so you must explain what medical or other serious factors have resulted in the delay – please refer to the guidance notes. Ask your supervisor to support the application in writing. It can take about three months to check your references, get the dissertation typed, proof-read, corrected and bound etc, so don't be over-optimistic.
If you are overrunning you also need to seek exemption from paying the University Composition fee. This form can also be obtained from CamSIS . As ever, it is a good idea to e-mail your supervisor once you have given them the form advising them where to send it; this ensures that should sudden illness ordisaster strike, you have a record of having completed your part of the process.
There are various sources of money available to you if you are running out of money on account of overrunning. These include College, and University hardship funds, departmental funds, the Cambridge Philosophical Society for scientists, and various prizes and awards. The Graduate Union can provide advice if you find yourself in this situation.