"... the language of the statute indicates that judges are expected, albeit starting from the assumption, to exercise their ability to reach a reasonable conclusion in the light of the information before them. It is to be noted that the assumption will be rebutted, if at all, as an exercise of judgment: the statute includes no reference to the burden or standard of proof. As we have indicated above, it will usually be unreasonable to conclude that the assumption applies unless information about the offences, pattern of behaviour and offender show a significant risk of serious harm from further offences."