Re: 164 fuel smell
There's lots of stuff on the American 164 site about fuel smells. I gather there's an inspection plate right at the leading edge of the boot floor, just behind the bulkhead. Check that it's sealed around the top of the gauge and fuel pump area.
Also, if it's round the rear wheelarch, have a good look at the filler neck in the wheelarch. Depending on the age of the car, there are a few small bore "breather" hoses clipped to the filler neck. At the top (probably completely hidden by a clump of mud) there is some sort of T piece. I think mine had disintegrated some years ago. Mine is a pre-cat 12 valve so I don't know how the plumbing worked on the cat cars - there might be a rollover valve and / or a carbon can somewhere in that setup.
Also have a look under the car at the big hose clips that connect the filler neck to the short length of large bore hose going into the tank. By some inspired stroke genius, someone had put clips on my car with stainless bands and mild steel screws. They looked fine but inside the housing, the screw had just rusted away and wasn't gripping the band any more.