This is absolutely not true, because unscrupulous parties are careful to complain vociferously (and shamelessly) even about coverage that is heavily skewed in their own favour.

Since the Andrew Gilligan debacle (where the BBC sacked a reporter and apologised profusely over the self-evident truth he had reported: that Tony Blair lied about Iraqi WMD), the BBC has gradually transformed itself from a public broadcaster to a state broadcaster. The only time you hear a reasonable balance of views on an issue these days is where the governing party itself is split on it.