why delays in sentencing?

[GSD4ME]

A propos this dreadful case of Baby P, I read that the accused will be sentenced on December 15th.
WHy is this? I would have thought that the judge would know what sort of sentences were available for a particular crime and so if the accused were found guilty of crime A, B or C, then the sentence would be X, Y or Z.
So why is there a delay of over a month before sentence can be passed? Other cases seem to be able to have a sentence passed the same day.

[Ethel]

Most crimes of this seriousness have sentencing deferred - it's nothing unusual.

[sara3]

generally cases are adjourned for sentencing whilst pre-sentence reports are sought from the probation service, and possibly medical reports.

[Barmaid]

Apart from murder where the sentence is often passed immediately, there is normally a delay in sentencing of at least 3 weeks for reports as Sara says. Other factors may come into play such as the Judge has reserved the sentencing to himself (normal after he has heard the trial) and he may not be sitting again until then.