I worked at ELEXON full-time (More than a year)
Nice area to work in, basic extras
High staff turnover, management not very supportive
I applied online. The process took 2+ months. I interviewed at ELEXON (London, England (UK)) in September-2014.
Assessment day. Broken down into a group task, individual pre prepared presentation and also a competency interview. Two managers interviewed me, all pretty standard questions (why elexon, why you, why the role etc.) Lasted around 40 minutes.