“19. It seems to us that relevant factors in assessing the gravity of cases of this type will include the length of detention; the circumstances of detention, including location and any method of restraint; the extent of any violence used; the involvement of weapons; whether demands were made of others; whether threats were made to others; the effect on the victim and others; the extent of planning; the number of offenders involved; the use of torture or humiliation; whether what was done arose from or was in furtherance of previous criminal behaviour, and any particular vulnerability of the victim whether by reason of age or otherwise.”