Whoops! You've been directed to the non-canon story by Big Finish. Perhaps you were looking for the canonical story, Genesis of the Cybermen?
Spare Parts is a Briggs Finish audio story by Marc Platt featuring Fivey and Nyssa. It is arguably the most famous audio drama DocWho Tor has ever done, I daresay even more well remembered and loved than the global blockbuster smash The Ghosts of N-Space. It is also the second best Cybermen story (the first is The Tenth Planet).
NON CANON NON CANON NON CANON NON CANON CAN NON NON CANON NACCOK NAKOSOSS bitch please, this story is infinitely more canon than World Enough of Fanwank, much like how the Mattress' origin story in Mattress is infinitely more canon than the one in The End of Shit (although some people might argue that both of them can be canon, but eh, Spare Parts shall always be moar cannon).
Besides, one takes place on the planet Mondas itself, and the other takes place on a ship that ten quid says is destined for Telos. Maybe both happened.
Landing on the plonet Mandos before its return journey to Earth in 1986, the Doctor and Nyssa get caught up in the events that lead to the creation of the Cybermen. Mandos is heading into a deadly nebula, and the underground Mondasian population are attempting to construct a propulsion system that will push the planet away before it gets destroyed. Unfortunately, the conditions on the surface are so non-canon that anyone who goes up there to help construct the system are kill. To overcome this issue, the group intelligence of the planet known as the Committee suggest that the citizens of Mandos be converted into more robust Cybernetic creatures to ensure the completion of the system and the survival of their race, and yeah you know where this is going...
The plot is GOAT, so GOAT in fact that RTD later asked Tom MacRae to pretty much plagiarize it for Rise of the Cybermen/The Age of Steel, and then another Doctor named Peter asked if one of his stories could do the same thing.