I had this
problem with mine , after fixing the leak which was the drivers door seal i had the carpets cleaned but a few days later the smell came back and got worse
I pulled the carpet back and and the soundproofing stunk ,it was dry but the smell was intoxicating

removing the underlay was my only option so i removed the hole interior which was extreme but easy if you take your time , sourcing new underlay for a leon was a
problem for me as none of my local breakers had any leons in so after a little research i got the underlay from a mk4 golf which is the same as a leon ,it fitted a treat (identical) after refitting it and a good carpet shampoo when it was out the
problem was solved