Read this with interest. Not the same as my current situation but I'm also currently trying to get money back from a trader, all be it the paltry sum of £168.
What I've discovered about these guys is that no matter how minted they are they will do everything they can to hold onto 2p if they think you can be fobbed off. They are competitive as hell and it gets that it's just about winning to them.
Play them at their own game, don't make it easy for them to pick your pocket. Go to SCC and follow all the good advice the other posters have given. I got advice from here on my problem and it's serving me well.