if my memory serves me well it was sung because we didnt know who he was as an unknown sub for man U - i think the goldstone faithful were trying to be ironic - its ironic that he turned out to be perhaps the most iconic footballer of the generation!
I was there for that game and I knew Beckham made his debut against us. It never clicked that I saw his debut though.
Anyway, here is the fresh-faced Becks at The Goldstone: