Submission
The first step is to notify the university of your "Intention to Submit" by completing the appropriate form.
Hand in the form (or email a scan of it) to the MPS School Office on mps-pgrsupport@sussex.ac.uk who will forward it to the Research Student Admin office in Sussex House. The form must be accompanied by a one-page Abstract of the thesis.
You can find the "Intention to Submit" form on the Research Student Administration Office's webpage. It is important to read this page because it gives detailed guidance on how to prepare and lay out the thesis. You can also find an explanation of how the viva examination process works, and related matters such as copyright and embargoes.
For Distant viva advice, please see the updated university's guidance here.
The university's formal statement of requirements are in Section 10 of the Handbook & Regulations for Doctoral Researchers.