What tune does it use?
The Sleigh Ride (or Troika from Prokofiev‘s score for the 1934 film Lieutenant Kijé.
How does it work?
Lake lifts the entire melody and uses it at the end of the first verse, from 0’50”:
Then after the second verse the tune appears once more :
Finally after the third verse we hear it again, by which time the song has built through a crescendo with a sizable orchestra and chorus:
Here is the original:
and here it is when the tune really gets going:
What else is new?
Prokofiev is a popular composer for lovers of pop music, and it is quite possible you hear his music more often than you realise. His Dance of the Knights from Romeo and Juliet, for instance, has been used as the theme for the BBC show The Apprentice since it started in 2005:
Greg Lake, of course, is no stranger to arranging or manipulating classical music. Here is arguably his most famous piece of work in that area, the rocking out of Copland’s Fanfare for the Common Man as part of Emerson, Lake & Palmer: