If you search the forum for 'interview' there are lots of threads giving advice about PhD interviews generally, which should be useful.
One additional tip - I interviewed (successfully) for a DTC studentship, and one thing they asked was why I wanted to do my PhD with a DTC rather than the more traditional route, so it might be worth knowing what the difference is, what DTCs are all about and why you think they're a good thing/suitable for you.