My advice is simple - don't do it, at least until you've done a forum search on the subject.
Click on the word 'classic' in your post & scroll down to the subject.
Certs can be had following an inspection of a DIY installation by a member of the UKLPGA and a record made on the UKLPG register (which can only be done by a UKLPGA member).....
(as can be seen I have been reading my copy of the UKLPG Code Of Practise for Autogas installations )