Ok, so I’ve just found out how to use Chromecast to play local video’s from my PC, using a Google Chrome tab (rather than the experimental cast entire screen option that makes the video choppy).
The trick is NOT TO USE REALPLAYER CLOUD! Instead, install any media player that has a Google Chrome plugin that plays video within a tab. My favourite is VLC Player. Then open Google Chrome and type the file path into the address bar in the following format (note the direction of the slashes!):
file:///E:/Media/Films/Avengers,%20The.mkv
You will then be able to play local video from your PC straight to Chromecast without having to install RealPlayer Cloud!