Well, you do 'just buy the certs' but only if your registered, when I purchase my certs I have to supply my unique installer number, the cert (which are also uniquely numbered) are issued against my installer number and therefore traceable back to me in the event of something done/going wrong.
As regards asking someone else to come and check/certify his work, I wouldn't personally for a number of reasons, including, once I check and certify I have know just taken responsibility for his work and any problems from that moment on are mine not his! also I have just enabled a non-registered installer to continue to work without the required certification (that I had to pay for, training, membership etc.)
If I was you, I would contact rgii in the morning and ask them about it as they are the body charged with regulation of the gas industry
I would with hold payment until this issue is sorted out, you have no guarantee of anything, let alone the standards, would be possible for the manufacturer to also void the warranty on the appliance