Hi! I went to one of these last week, and it was quite similar to Tilly 112's experience, except that we didn't have to approach customers or anything like that. There were about 40 of us in a room, split into groups of 5/6, then we had to watch the powerpoint, and do 3 tasks - 1 was presenting yourself to the group for 2 minutes, another task was designing a John Lewis poster, and lastly we had to think of the qualities which would make up a good employee. They also had the 1 to 1 interview (lasting literally 2 minutes and mostly about hours/other practicalities). I don't think you can really prepare for it - but then again yours could be different. You simply have to show you deal well with people, are friendly, considerate, etc. Oh, and most people wore 'work' clothes - so something similar to what you would wear if you were working there; shirt and tie sounds good.
And they let us know the results of the interview the next day!