Any instrument purporting--
(a) to be an instrument issued, made or given for the purposes of this Act, and
(b) to have been signed by the person authorised to issue, make or give the instrument, or by another person acting as delegate or on behalf of the person,is admissible in any proceedings under this Act and (in the absence of evidence to the contrary) is to be taken to be such an instrument and to have been so signed.