First thing were the four or so pre-recorded interview questions online. These were as described below. You don't get long to answer these and it is definitely helpful to prepare in advance.
Then I received an invite to a face-to-face interview. I went to this and didn't get past this stage. It was a one on one interview at the London office (if you live far from the London office they offer the opportunity to do this stage as a google hangout).
I was slightly early, the interviewer was slightly late. It took approximately an hour.
I had to go over my CV in some detail - re my more recent experience.
If I had continued I was told that the next stages would be an exercise, which you are given the opportunity to prepare for in advance, then an interview with one of the founding partners - I think the COO, which I was impressed by.
The overall interview process feels very long what with 4 different stages but they are screening for the right type of person for their environment.