I wanted to clean this up a little, so here is a chart that tracks how each pollster tends to lean when calculating support levels for the various parties, as compared to the average polling results each month. This does not necessarily equate to a deliberate bias, but instead is more reflective of the polling methods used.
As you can see, the list is not complete yet.
The following chart shows each pollster's average variation from other polling firms. The numbers are the amount of percentage points a particular pollster favours or disfavours that particular party compared to other pollsters.