I took the online test on May 12th. Was told I passed that same day; CV review followed. Invitation to assessment centre received on the 15th. Assessment centre held 22nd.
The whole process began from about 9am and ended at around 3pm. There's some early on networking with your assessors, and then you're briefed on the day's schedule. The first part for me was the presentation and competency-based questions. Presentation should take about 10 minutes and then you're asked questions based on your presentation and CV, for the assessor to get a better understanding of you and your experiences. Next were questions about the company: Recent bad press, who are their customers, how do they currently sell their engines, challenges facing the company currently (economic, etc). This section ended with competency based questions along the lines of "Give an instance of when you had to influence someone or a group of people" and "a time you worked in a team and had to take a different approach to reaching goals". There are follow up questions and some advice on these scenarios based on your answers. At least there were in my case. This section is likely the easiest to cruise through.
Numerical test similar to online test, but now on paper. Not really challenging.
The group exercise was next and it was shocking, to be honest. You're tasked with choosing and constructing paper planes in your groups (of 5 or 6) and making a presentation at the end. Assessors are watching the whole period and take notes on each person. You're asked questions after the group presentation. It was shocking because there were overly aggressive members of the group who wouldn't give other people a chance to speak and would literally hijack suggestions people were making to make themselves appear to be the originators, or hijack tasks people had started on. Don't know how well that played out with the assessors, but it certainly ticked off group members. Try to keep to time throughout. You have about a half hour.
Finally, technical interview. Hands down the toughest part of the day. It starts off with generic "Why engineering?" or "Why Rolls-Royce?" or "What engineering experience do you have outside your coursework?" questions. Progresses to technical questions about failure modes, materials, functions, etc. You say what aspect of engineering you're studying and they ask you questions under that. I'm in Mechanical Engineering. The questions get progressively harder. I was unprepared for questions on pumps and floundered there, but most questions tie into aero engines so you want to be prepared for that. Talk extensively when answering and ask for clarification if you're unsure. My assessor here was alright for most of it, but he seemed exasperated at certain points. I'd been told that they want to see how you think and try to get to an answer, and sometimes guide you through. Nope. Didn't get that sense in my case. Just get the answer right.
At some point you get to ask people on the Graduate scheme some questions about the company. It's a chilled out session really, but you don't get any points for brown-nosing there so save your energy if you're the type.