There are seals on the tank under the rear seats where the fuel pump goes into the tank.. thus the smell in the boot.
I would suggest removing the rear seat (very easy, just pull) and checking the plastic cover. If you can smell fuel there then you might have a seal issue around the fuel pump or something similar
Also.. when you fill the car are you putting the pump into the hole on the side of your car? I find that actually pouring the fuel INTO the car makes a big difference and ends up much cheaper. Since I read that unlike plants, the car doesn't enjoy being watered, I have cut my fuel costs by over 2000% and no longer have fires on the car when I throw spliffs out of the window. Now I only put fuel in the car and not all over it.