I had an interview with them last cycle, my first ever and I was nervous. I, like you, had very little notice for this. It was with head of Grad Recruitment Paul Gascoyne and a partner. It was based a lot on my application - and the application questions. How Shearman maintains competitive advantage (which is still a question in the app I believe), Why Commercial law, why London. I also had a commercial interest question. Be confident and take your time. Maybe use the three point answer. Firstly, I believe...secondly etc.
Good luck and be yourself, let them get to know you. - I liked the firm a lot.