I have always just written them a letter, either for SGC or FAC and it has always been ok.
Simple handwritten letter.
Name, address, cert number
have purchased , borrowed, etc shot gun serial number XXXXXX single barrel 12g baikal from
rfd number, address on date xx/xx/08
or license holder
name, address cert number on date
signed
XXXXX
Then send to.
FAO firearms administration team at your constabulary that deals with it.
Thats all you need.
Matt