eviction gassed?

Possession - Section 8 and Section 21

I have a difficult tenant who I would like to move on.
What are my chances of a successful section 21 application for possession order?
The last ast was dated December 2018 and I have the tenant's signature for gas cert, epc, how to rent etc. I have that gas cert and the present one. And inbetween.
Here is the challenge ; this tenant began their occupation in this property with an original ast dated 29 05 2015. I have this ast but cannot find the gas cert for then. The first one I can find (with the help of my gas engineer) is dated 09 07 2015. So I cannot prove that I gave the tenant a gas cert before their original occupation.
Studying Barely Legal's flow chart he does not seem to think it is clear cut.
Should I take a punt and fill in the form N5b giving just the current ast with relevant certs, ignoring the first ast or is this pushing my luck? Or should I tell the whole story and put 2015 as the first tenancy raising the question of the cert.
I am sure I had a gas cert back in May 2015 but threw it away when the 2nd tenancy was signed.
I don't want to issue a s21 unless I can follow it through. I don't like idle threats. Unless I can complete N5b properly I don't want to start the process....