Well we got so many certificates for things that were done on the house that we could well have the one your talking about, but I would have to go through all the documents and see if it's in there. As for it having a blockage, we got someone out to clear it and it worked ok for a few days, then started gurgling and blocking again, even after deciding not to put toilet paper down the toilet. Only our 3 yr old lad has used that toilet since during his potty training. And yet the water still doesn't drain properly. But as I said, if I push the toilet towards the wall the water stayed in the bowl, pulled it out slightly and it disappeared as normal, but with a small leak on the floor. So I'm struggling to find the reason for all of this. I'm gonna try replacing that connector and see if anything improves. The pipe from the toilet doesn't look round though, should it be? And has got damaged or misshaped somehow?Yes lots of people use a flexible pan connector, I just told you how I would do it - but it’s not the only way!
As to venting the drain, could be a couple of days work. But are you sure it’s not just blocked somewhere?
If previous owner did it themselves - do you have the building control certificate, which would be a fair indicator that things had been done correctly? It is notifiable work…