Yeah adrian, history lesson: punk is American. You can trace it all the way back to the first Stooges album in 1969 (the song 1969 is pure punk in sound and attitude), or if your mind can't accept punk existing in the same year the Beatles broke up (ask your dad who they were), then you can go modern with the first Ramones album in 1976 (though actually, they were playing shows in 1975). No Ramones, no UK punk as you came to know and love it, period.
The only thing "punk" about Nirvana (and I loved Nirvana, don't get me wrong), was their rejection of 80's style. Musically there was not one iota of anything new there. But even established forms benefit from masterful interpretation, and Cobain was a master.
Oh look, someone poked the old man in his cage and he growled about "punk" again! Zzzzzzzz.
Carry on.