Two stages of interviews, both were very long days. First was Local, they give you a lot of "quizes" or tests. Usually its short arithmetic/pattern finding questions, they are timed. They don't expect you to get everything but it does take a huge amount of time. Then they give you a longer problem, like a puzzle type question, they gave me two, they are sort of real life problems with logical solutions. So I had one where they were sending messages in medieval england using messengers/horses/ravens and you had to write rules for how to send what message. I then had a second interview in London, this was quite similar in formal except the longer puzzles are different, you have to give a presentation which you prepared before hand on anything subject you want. You also get a chance to meet a recent graduate hire to talk to them. I didn't make it through this stage. I would suggest that you shouldn't worry about solving all the puzzles, try your best but be very receptive when they try and guide you. Be prepared for a long day, it will be tiring and you need the energy for the puzzles. The people are very nice and polite, the interview process is very rigorous and takes quite a long time. The days are very tiring and travel can be hard as well.