I applied online. I interviewed at Mars (Slough, Berkshire, South East England, England) in Dec 2015
Interview
First was the online application form, which included some long answer questions like Why Mars.
Then came a telephone interview with similar intro questions like why this company, followed by competency questions including 'tell me about a difficult piece of work you've completed' and 'tell me about a difficult working relationship'.
After that was an invite to a two-day assessment centre, but not everyone advanced to the second day. The first day included a group exercise looking at business scenarios. Then a pre-prepared 10 minute presentation on the topic of '3 problems the FMCG industry face', followed by questions on the presentation and more competency-based questions.
I didn't progress to the second day, so can't say what that involves, except you didn't have to prepare anything for it.