Tom, what I did was go through magazines like Cinefex, Broadcast and Creative Review, make a list of all the companies that advertise or are featured and called them up. These days (it was a long time ago that I did this) you could just do a Google search for "London Post Production Facilities" or "London Visual Effects". Most companies aren't going to advertise running jobs because there will always be a stream of people calling up and asking for them. The trick is to be the person who calls up on the morning when the company realises that they need a runner and that's why it's best to find a lot of companies and call them reasonably regularly. With big companies like DNeg, MPC, Cinesite and Framestore it's best to keep applying online but you will need a reel of a fair standard to be considered. Hope that helps a bit.
