I'm afraid that's not correct about Purley.
It was (and still is) not possible to access the yard (gravel plant) directly from the main line, the only access is from the down siding. The running lines are far too busy to allow shunting to take place, even if it was possible.
Trains normally arrive in the down direction into platform 6, the loco runs round, then draws the train into down siding. Once the train is in the siding all of the shunting takes place from there. When the shunting is complete, the whole train including the loco is contained within the gravel plant as the siding is sometimes used for EMU's if required.