Section 21 Stop and question
A member of Her Majesty's forces on duty or a constable may stop a person for so long as is necessary to question him to ascertain his identity and movements.
A member of Her Majesty's forces on duty may stop a person for so long as is necessary to question him to ascertain—
what he knows about a recent explosion or another recent incident endangering life;
what he knows about a person killed or injured in a recent explosion or incident.
A person commits an offence if he—
fails to stop when required to do so under this section,
refuses to answer a question addressed to him under this section, or
fails to answer to the best of his knowledge and ability a question addressed to him under this section.
A person guilty of an offence under this section shall be liable on summary conviction to a fine not exceeding level 5 on the standard scale.
A power to stop a person under this section includes a power to stop a vehicle (other than an aircraft which is airborne).