I had an interview a few years back and it went as follows:
Big group (about 20 people) in a conference room sat around the table. We had to introduce ourselves and give a briefing on who we were and our background. Then we did a test where they gave us scenarios about work and asked us what we would do in those situations. Things like two VS aircraft about to land and only one can get a jetty stand. Which one would you choose & why. After that we were split into two groups. They gave us a topic and we had to jointly develop a 10 minute presentation & then present it. Think the whole thing took about 3 hours or so. We were then sent home. After that they called you back for an interview with 3 people from VS. That was a more formal interview with the OPS Manager, a VS Duty manager & someone from HR. From there they chose who got the jobs.
Also there is a LHR based VS TCO who normally frequents these boards. Hopefully he will see this and give you some inside information