In case anyone has an interview coming up and is curious about what happens, I'm writing a detailed account of the day.
The admissions room was on the 7th floor, and it gradually filled up with about 30 to 40 people from 10:00 to 10:30. Everyone was sitting in nervous silence waiting for the admissions tutor to show up. Once the admissions staff had arrived there was a brief introduction and then they registered us "with a twist". This involved telling the group your first name, where you're from and why you want to study mechanical engineering. It wasn't especially intimidating, and most answers were fairly similar. If you're reading this then you'll have an advantage over most of the students who aren't expecting it.
Next was a powerpoint presentation (~45 minutes) about engineering in general, which dealt with the responsibilities of engineers. After this they gave us a voucher for tea/coffee which we got from a canteen down the corridor. Then there was a presentation about the structure of the course and the options available, and we were given a simple application form which could be filled in later.
Then lunch, which was in a small room with a table full of sandwiches. A lot of people, including myself, didn't really eat anything and started talking to the current students straight away. It's a nice opportunity to ask questions and get a bit more knowledge about the interview. At 13:00 our attention was drawn to an interview timetable which showed us the details of the interview. The interview times varied between 14:15 and 15:30, I had one of the early slots because I lived about 3 hours away. In the time before the interview we were shown around the department by two current undergraduates, it would have been a lot more enjoyable if our mind wasn't on the impending interview, but it's still a good (but fairly unorganised) tour.
We returned to the initial room about 10 minutes before our interviews, and it was up to us to scatter ourselves and find the interviewer.
My interview was on the 6th floor, so not very difficult to find. It started with him handing me an engine component from one of his shelves, which I had to describe. I knew it was a cam-shaft which drives the intake/outtake valves on an engine, so I started off fairly confidently, although I forgot about the bearing and it took me a while to point out where the drive belt goes. He then asked me about living in London, which is an easy question, just be positive about it. My lack of work-experience was picked up on, but in the end he seemed fairly positive about my application and he told me I'd need 3 As to get in. It was over very briefly (so only about 12 minutes long).
It was implied that I'd receive an offer, but the interview wasn't fantastic, so I'm not sure yet. All in all, it wasn't a very difficult day, although if you have to get up at 5:30 like I did you will be quite tired by the end.