It is a great song but not historically correct.

What actually happened was, that the British attacked three or more times, and when they finally gave up it was only 22 soldiers who actually RAN.

Not quite the rout that the song suggests.

Also, as an aside, Packenham was Wellington's Brother in Law and they didn't send Wellington, because Wellington was pro-American.

Wellington would never have camped where Packenham did and was far too experienced a soldier to have attacked under those circumstances.

I like to think that Wellington was visionary enough to see what a great ally America would be in the future.