I had a petrol smell in the car for 6 months before I found the problem - tried everything (almost).
I decided to get serious about the vapour breather pipes behind the panel above the wheel arch. In fact, I removed the rear wing so that I see if I had a blocked or broken pipe.
Here's what solved it.
The plastic pipe exits the boot area inside a rubber grommet protecting the pipe from rubbing on the rough metal of the hole. The grommet had perished and the plastic pipe had rubbed itself a hole on the unprotected metal hole. The fumes where able to flow back into the boot, behind the painted metal panel, and into the cabin through a hole I found behind the back seat high up near where the rear glass and the metal bulkhead meet.
I filled the hole with metal epoxy glue being careful not to cut off the fume flow, cut a slot in a new rubber grommet, fitted the grommet over the pipe with slot side up, squirted black silicone sealant in that area. I also filled the gap I found behind the rear seat.
I haven't had the fume smell since and it's been over a year.
There are other causes of a fume smell but that was my experience.
Eric