Is there a requirement in law to carry any of these documents?
23.8 Section 48 of the 1968 Act provides that a constable may demand from any person believed to be in possession of a firearm or ammunition to which section 1 of the 1968 Act applies, or of a shot gun, the production of their firearm certificate, or, as the case may be, their shot gun certificate. Section 48(2) of the 1968 Act provides a power to seize weapons and ammunition in cases of non-compliance.
I have a photo stat of it that I keep in the pouch on my gun slip which i roll up and place in a locker