F1SI 1985/454
21(1)The returning officer shall as soon as practicable send to those entitled to vote by post, at the addresses[F2 shown in the absent voters list], a ballot paper and a declaration of identity in the[F3 relevant] form prescribed in the Appendix, together with an envelope for their return.
(2)The declaration of identity shall be witnessed by another elector who personally knows the voter.