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.

They have an online interview with maths and puzzle questions and some coding in Ruby. Later their is an in person interview that includes more puzzle/logical questions and then a pseudo programming test with a discussion about answers and scalability of solutions.