Professor Bart Comments of the London School of Economics did research on this and found that 75% of the media coverage of Corbyn misrepresents him

We all want and need a strong and a critical media, but maybe we do not need an attack dog that kills off anyone who challenges the status quo

If the media was more balanced its reporting of Labour would Corbyn be winning now? I think so.

Our rigorous and statistically representative analysis concluded that when it comes to the coverage of Corbyn in his role as leader of the opposition, the majority of the press did not act as a critical watchdog of the powers that be, but rather more often as an antagonistic attackdog. 

Over half of the news articles were critical or antagonistic in tone, compared to two thirds of all editorials and opinion pieces. Besides the almost total lack of support in the latter, especially in the rightwing media, the high level of negativity in the news reporting struck us as noteworthy here. According to the Independent Press Standards Organization (IPSO), newspapers are obliged to ‘make a clear distinction between comment, conjecture and fact’ and this also did not apply to Corbyn. Furthermore, Corbyn’s voice is often absent in the reporting on him, and when it is present it is often presented in a highly distorted way. In terms of the news sources used in the articles, the civil war within Labour is very enthusiastically amplified. In most newspapers, including The Daily Mirror and The Independent, Labour voices that are anti-Corbyn outweigh those that are pro-Corbyn. 

In addition to this, a prevalent way to deride Corbyn is through scorn and ridicule. Three in ten news stories, opinion pieces, or letters to the editor mock Corbyn or scoff at his ideas, his personal life, his looks and/or his lifestyle. Besides these character assassinations, some of the popular mantras repeated over and over again in connection with Corbyn are: that he is unelectable, that his ideas are unrealistic and loony, and that he is unpatriotic. 

http://www.independent.co.uk/voices/jeremy-corbyn-media-bias-labour-mainstream-press-lse-study-misrepresentation-we-cant-ignore-bias-a7144381.html