Richard - I suspect you were referring to the Tale of the Christmas Tree Fairy, penned by Tim Brooks and Mick Fall (then students at Imperial College) in 1972:

I'll tell you an old Christmas story,
As we sit round the log fire at night
Why each Christmas Tree has a fairy on top
And why Santa's beard is so white.

I've e-mailed Tim, so that he's aware of this thread....

Kitty