Multiple rounds of tests for aptitude. On multiple days. Basic coding knowledge helpful but not required as they accept people with the skills they are looking for. I interviewed a few years ago so things might have changed

The interview process involves going through a day of tests for which you dont get told what they are. One was a high IQ test (to justify their arrogance) One was a verbal reasoning One was a basic code review challenge Then there was an interview with a developer. He told me that I would want to work in finance, when finance was not even on my CV! He seemed upset he didnt work in finance. He was very aggressive and lacked social skills at the same time. Noticeably the office was VERY MALE which was very off putting. All the women seemed to have low level admin jobs which was VERY OFFPUTTING. Seem to have a fascination with maths grads. The lady from HR phoned me after to tell me that they had rejected a PhD Physics graduate from Oxford recently for not passing the hiring bar. She delighted into telling me this.

An on-site interview consisting of one round of multiple choice mental maths questions and second round of 8 math/algorithmic puzzles followed by an interview discussing the answers. This final interview also asks more genral fit questions.