The 'left' think the BBC reporting is too much to the 'right', and the 'right' think the BBC reporting is too much to the 'left'.
The BBC balance in reporting is doing its job well.
TBH, I've always thought that was a strange argument to make in defence of the supposed lack of BBC bias. If everyone involved in an event believes that the situation and their position on it has been mischaracterised or misreported, that's not a sign that the BBC "is doing its job well".
I'm not convinced that the BBC is either pro-left or pro-right wing or pro-Tories or pro-Labour as what constitutes "left wing" and "right wing" changes over the years as do the policies of the Tories and Labour. The Tories introduced Section 28 in the 1980s but later legislated for gay marriage. The Labour Party (the LPP at least) was pro-Iraq War, the current leadership has said that it was a mistake and apologised for it.
If the BBC is anything, it is pro-Establishment. The BBC generally supports the direction of travel of the institutions of government and the intellectual underpinnings thereof. That's why it is a priorty to broadcast television channels in Arabic and in Persian and why there are pushes to increase Russian coverage and introduce North Korean coverage - because it is a priority for the Establishment to get their message across.
It also acutely feels the need to fairly and positively represent the diversity of the UK because we all pay for the BBC. Personally, I have always like the Beeb because of this however the BBC will naturally err towards the arguments and viewpoints of the people and organisations that it feels represents our nation's diversity. I think that is why immigration has always been a particularly difficult area for it to cover without it stepping on someone's toes - put it this way, I wouldn't want to be in charge of it as it seems like an impossible task.
OFCOM's queen bitch