The first proper quantitative analysis of BBC output examining their political bias, or not, has been done by an academic.
In the past the evidence has merely been anecdotal and therefore open to challenge.
The conclusion is clear - the BBC has a very clear left wing slant in its output that resembles the Guardian a lot more than The Telegraph.
This is a problem because the BBC charter requires it to be unbiased and it is funded via a poll tax.
Here is a blog about the analysis. No doubt the full report will get a lot of coverage.
http://www.cps.org.uk/blog/q/date/2013/08/15/bias-at-the-beeb-addressing-some-questions/