It's worth noting that the linked article does offer selection bias as an explanation – suggesting that conservative supporters were more likely to write letters, supporters of other parties less likely to.
Number of complaints seems a pretty poor measure of bias, but it is one that is often used to reinforce the idea of the BBC's impartiality.
Well, as someone who doesn't support any of the UK major national parties I'd say that the BBC was fairly good at making pretty much all of them look equally unappealing!
Number of complaints seems a pretty poor measure of bias, but it is one that is often used to reinforce the idea of the BBC's impartiality.