Backpacker you are right that the fuel cap was relocated for the XS.
I remember the XSs still smelt though - don't know why but it was pretty strong.
There was a mod early on regarding replacement of the fuel pipe within the fuselage as the original pipes weren't up to the job.
I had a suspicion that there was some porous material somewhere behind the problem, maybe the tank material, maybe the fuel hoses, maybe it was leakage around the fuel filters which are under the seats.
Europa are great little aircraft though - I would have paid a fortune and flown more slowly in an average certified airframe.
Right now it makes more sense for me to fly certified (night/IFR) but when I retire I may well go back to LAA as they make so much more financial sense.