Submitted the online application form in early August, and within a week was sent an online Situational Judgement Test. You have 7 days to complete it, but only takes 45 min, and it's easy enough when avoiding passive aggressive answers.
About a month later (September) Atkins emailed that I had passed the SJT, and asked for more CV details, which at this point were being analyzed by a real person.
By October they sent online numerical and inductive reasoning tests, same thing, you have 7 days to complete both, which are timed to 25 minutes and pretty difficult to finish on time. Try to do some practice tests beforehand to understand the format. I don't think I scored above 80% on either, but instantly received word that I was through to the next stage: the telephone interview.
This bit never actually happened. I think it's just to check that you are polite and speak English fluently, because after exchanging some emails with the recruiter, they decided to skip the phone interview and invited me directly to the assessment centre in November.
No need to study for the Assessment if you know the basics of the area you applied for, and even then, there's a diverse group of people applying for the same job from different majors. Bring a calculator if you have one for the technical tests. Also, the recruiters are there when you are with the other candidates on break, etc, so be friendly to everyone and try to put yourself and others at ease. Don't behave like it's a competition, since you'll probably have some on your group exercise team later on.
The 30 min group exercise is key, because it's the most realistic scenario of how you behave when trying to solve a work problem, and some candidates (I would think) disqualify themselves instantly by being overly stubborn on the problem, or by not being able to get a coherent word into the discussion. Try to keep the discussion organized, remind everyone what the end goal is, and try not to ramble on a singular topic for the entire duration of the exercise.
The one on one interview is mostly going over your CV, and telling you which projects you might be working on if hired.
Received word that I had an offer within 7 days of assessment.