Initially, they used an agency to conduct a phone interview with candidates, addressing some competency questions. Having passed that hurdle, 52 (!) candidates attended an assessment centre which involved typical group exercises (you're stranded in a desert, what objects should we take?) as well as 3 minute individual presentations and - a lot - of waiting. The agency (a subsidiary of Randstad) clearly treated candidates like idiots, reminding us to wear a suit and shake hands and kept calling to check you knew what you were meant to be doing. They were very keen to make sure you would take a potential offer and no flexibility whatsoever regarding the starting date.
Following this day there was an assessment centre at Dimension Data itself with the 18 best candidates, which was run in a much better fashion. This was at their office in Fleet, and involved group presentations, a written exercise as well as individual interviews. They said they'd pay for travel to their offices but I still haven't received the money!