Very interesting! I'm not sure if this explains some of my problems or not but very interesting that it happened on around the same time as my search traffic declined.
I also run another site with a similar amount of traffic in a different industry that has been uneffected. I have dilligently tried to create good content and generally good link building practices for both sites but one has been punished and the other not.
One site is a hotel website with twice weekly blog posts and the other (with the problems) is more actively changing - a blog based on renewable energy/solar power/innovation/design etc with new posts every 2nd day.
I'll be continuing to build good content for both sites while scratching my head as to where I went wrong here + hope things will turn around...