“It is not merely of some importance, but of fundamental importance that justice should not only be done, but should manifestly and undoubtedly be seen to be done.” (R v Sussex Justices, ex p McCarthy  1 KB 256.15).
In essence, the rule against bias means that the decision-maker should not have an interest in the outcome of the decision.
A decision-maker will have a bias if its attitude in making a decision prevents it from having an open mind and from making an objective decision.
The Court will quash a public authority’s decision if the decision-maker has a direct bias (i.e. where it has an interest in the decision being made).
The Court may also quash the decision where the public authority has the appearance of bias. The court will find the appearance of bias where a fair minded and informed observer, having considered the facts, would conclude that there was a real possibility that the decision-maker was biased.