Not many people were aware of the offence "misconduct in public office" and fewer still were aware that the maximum penalty is life imprisonment. But the details are readily available on the Crown Prosecution Service (CPS) website and were updated only in November last year.
Although the maximum penalty is life imprisonment, the CPS makes it clear that a statutory offence should always be used instead, if there is one. The CPS advice does not go into detail, but in the case of Damian Green, the Official Secrets Act was the relevant statute.