I had an interview for SPS, at Cambridge, and am a mature student. I was asked some questions about my personal statement (I had mentioned nature/nurture, whether psychology was a science, some stuff about why people commit crimes etc). I was also asked some separate questions, about stuff I had never really thought about before. I was asked to argue something one way, and then the other. There were also some practical questions about my access needs, as a disabled student. If you have kids or other practical considerations, they may ask about those things.
Make sure you have recast your eye over your HND work, know the course you are applying for well (e.g. which bits appeal particularly). You shouldn't need to do much else. Good luck.