Either way it is worth getting in touch with your course/module leader if you're concerned about failing your dissertation - you've not mentioned anything about your circumstances but I'm guessing that if you've passed everything else on your course, there may be extenuating circumstances around your dissertation submission. If so, this could allow you to either extend your submission deadline or, potentially, to resubmit.
Hope that helps.
If you can get a diploma, you should be capable of passing a disso. A dissertation is (arguably) harder then taught modules but to get 50% really should not be too tough. Just pass the disso & you dont have to worry about these things.
Why do you think you'll fail your dissertation?
I have already failed my dissertation. When I contacted the uni they told me that my award was based on passing year one but i also passed all my assignments in year two but not the final dissertation in year 2. So i don't see how they can only award me a certificate.
They would likely offer you the best award possible.