I applied online. The process took 1 day. I interviewed at Research Partnership (London, England) in Jun 2017
Interview
First was an online application, followed by 20 minute Skype Interview asking why you want to work for them etc. This (is successful) was followed by an assessment day. Assessment day included 2 group tasks, individual written market research tasks, a pre-prepared presentation on a controversial issue and a face-to-face interview. Tiring day but everyone is nice. Don't worry if there's not enough time for the written tasks - just have a go.