So, a little frustrating update that almost made me scream every swear word known to mankind over and over, but I took a deep breath and didn't because undoing the cover today after he drove for 100km the last 2 days revealed absolutely no fuel on top of the tank. Nothing, nada, zip, zero and I mean not even a hint of a smell when usually we would have it trickling down the side of the tank.
I mean yes, my sons over the moon with the result but as the one chasing the problem has left me scratching my head only to conclude that it was the floor of the car being applying pressure on top of the tank. Out of the car it wouldn't leak when full of fuel.
So, I have told him that I will be convinced after he has driven to work for a few days and has kept the tank as full as he can in that time, then we will check again on Wednesday.
If anyone has some thoughts, please feel free to share because I'm happy yet confused. I have chased this leak everywhere, and it just kept leaking and then it just stopped.
A quick spot of info about the install of the cover is that no, you don't have to do anything to the underside of the seat and no you don't feel it sitting on the seat. Saw people asking these questions on YouTube and never got answered. The quality of this kit that I got made everything seal up perfectly and looks professional when done.