Hi Poppy,
I wouldn't worry about no news, academics are not famed for their speed of response and the HR at my uni is laughably bad. A follow up "prod" is perfectly reasonable, however I would wait until at least a week or two has passed. It may take them that long to interview others, so unless you know that the process is over (e.g. they interviewed everyone on the same day or you are the only applicant) I would wait 2 weeks from the date of your interview before sending a polite email to ask when you will find out.
If you really can't wait then maybe contact someone in charge of the admin (supervisors secretary?) just to ask how long the wait might be and how they will let you know. You could put that you meant to ask at the interview.
Good luck, I hope it comes back a positive! (up)
All the best