This case occurred when the Home Secretary could still determine the length of sentences of prisoners, and he sentenced the killers of Jamie Bulger initially to 8 years and then, on the recommendation of the Lord Chief Justice, to 10 years.
Following a massive media campaign he moved it to 15 years.
HL said that:
The Home Secretary had failed to have regard to the development of the child along with deterrence, risk, retribution, etc. and that therefore his decision was unlawful.
That, in exercising a judicial function, the Home Secretary had to be detached from public opinion and make a decision without regard to irrelevant factors such as public opinion.
Said that if a Home Secretary is to take on a sentencing power then he must exercise it in the same way as a judge.
Said that in undertaking to breach separation of powers, Parliament would have intended the Home Secretary to act like a judge and indeed the Home Office stated that.
A judge, unlike the Home Secretary, would have ignored a newspaper campaign and thus the Home Secretary mad a decision that was not dispassionate and was therefore unlawful.