I don't go to Nottingham, but I do go to Birmingham -and we use WebCT and Turnitin. If all you're worried about is your draft dissertation causing your final version to be flagged as 'plagiarism', then you don't need to be. The main reason is that turnitin does not decide that an essay has been plagiarised: the worst that will happen is that it will flag your essay as having a high similarity rating and a marker will be aware of this and compare the two documents. They will then realise what's happened! Also, if it happens to you, it'll surely happen to everyone who submits a dissertation. Are you sure that you need to submit it twice, though? We only had to put our final through turnitin, given that the draft doesn't count towards your overall classification, I don't see why they'd care whether or not it was plagiarised as you would get found out in the final version, which is the one that matters.