Perhaps not the best one to give advice, as I had an interview for a PhD studentship but was not offered the funding (although they liked what I said and offered me a place at the uni., but I couldn't afford it.) My husband was successful in getting a studentship and offered me the advice, though!
If you know the other people on the research panel, try to investigate a bit more their areas of research, I appreciate you've said you already research in this area, but trying to link back in to what they do and how your work would link into current projects might help. My husband also said to ask if there is additional funding with regards to attending professional conferences, presenting papers, etc., or if your PhD would require any travel (sounds unlikely-mine was for rural internet users in Wales and had NO extra funding for travel, so it would have been an expensive undertaking.) Try to read up as much as you can into new areas, projects, ideas that are coming out of there to give your proposal a good side of things, too.
I was in front of a panel of 4 and I was only completely confused by one question, but I tried to stay calm and asked them to rephrase it until I understood it-I think I just blanked.
Again, I didn't get it, but it was a good experience!