I think it's highly unlikely that anyone will give you a difficult technical problem to solve. When I had my PhD interview (UK - chemical engineering) I came prepared for the hard technical questions, but I was woefully underprepared for the soft general questions like:
"Why do you want to do a PhD?"
"Where do you see yourself in 5 years?"
Aside from the soft stuff, the only other things I was quizzed about were my previous experience, and my general understanding of chemical engineering. They are not there to give you a test, they don't have the time or will. That's what university is for! They are there to assess your interest in your subject, your commitment to completing the PhD and whether or not you will 'fit in' with the research group.
PS: It is extremely likely that your final year project will be discussed at length as it is your only real research experience (unless you have industrial R&D experience). So be warned, and good luck!