best bit of advice i can give after being on about 5 shop interviews in my life is LEARN ABOUT THE COMPANY
checked and river island have a 'about us' section check that out..
some interviews ive been to have basically just been a friendly chat (got that job)
and others have been a proper Q+A (got that job too)
also look at the careers section and use their points to describe yourself soo for example the 'who we want' section learn that and use that to describe you..
I am obviously very interested in fashion and feel your company is one of the leading high street brands in providing the customer with the latest fashions available. I am enthsiastic about any challenge handed to me and enjoy working towards targets. Customers are of the upmost importance to me and feel I am approchable and can help with any problem given.
etc etc you get the idea