The petrol tank is under the car and the fuel lines run under the car, so in theory there's no way that petrol can leak into the cabin from the tank or it's pipes. That's the good news..
The fuel pump however, sits on top of the tank so they put a hatch to allow access to it, inside the car. It's on the floor under the rear seat cushions, so take them out and have a look. If fuel is leaking out of the fuel pump seal, then fuel will collect on top of the tank (in the pump recess) and vapour will get into the car, even though the hatch should keep the vapour out.
Having said that, the fuel filler
is technically inside the body of the car and there may not be (I didn't look behind the wheel arch liner) a continuous sealed surface between the petrol filler neck and the cabin.
So, it's also possible that the "escaped" petrol is getting into the cabin through the inside of the wing. Remember that petrol may be getting in as a vapour
that is evaporating off a surface... then mixing with moisture and condensing into your footwells.. so it's not necessarily a drip or a "leak" ... but usually petrol leaves a stain where it leaks.
Look at the tank (if it's leaking at the pump) and look at the filler pipes.
My bet is the fuel neck filler pipes or the re-circulation vent mechanism (vapour escaping from the tank has to go back into the tank, not to atmosphere.. for pollution regulations) have a leaky joint and fuel or vapour is getting out.. running down the pipe and then evaporating off and getting into the cabin.
The whole lot there is just flexible plastic pipes slotted together and clamped to the fuel tank under the car. When you fill the tank, some petrol will be trying to get out at the joints.. so apart from anything obvious (e.g. a detached vent pipe).. check all the joints.
Some of the rubber tubes are connected with Klik clips. Make sure they're tight and that none are missing.
The vent hose may or may not have a filter on the end. Make sure it's there and that it's correctly routed.
Take the rear wheel arch liner off and get your head in there.. you should easily spot it, judging by the amount of fuel you're finding. It will also help if you fill the tank up.. since if it's going to leak, it's more likely when the tank is full.. but if you mechanic has to dismantle/replace the filler neck tubes, make sure he doesn't nick any of it..