19.—(1) The use of force by a person in effecting or assisting in a lawful arrest, if only such as is reasonable in the circumstances as he or she believes them to be, does not constitute an offence.

(2) “use of force” in subsection (1) is defined and extended by section 20.

(3) For the purposes of this section the question as to whether the arrest is lawful shall be determined according to the circumstances as the person using the force believed them to be.