There isn't a single part number for the seat as the same parts are shared across the GTI & R32 for both the cloth and leather versions.
Here's the ETKA diagram and parts list for the Recaro seats
[pdf]http://www.mk5golfgti.co.uk/forum/downloads/partslist.pdf[/pdf]
[pdf]http://www.mk5golfgti.co.uk/forum/downloads/picturerecaro.pdf[/pdf]