hi there chaps!
i have a friend who has both pinion 50/37 for a longer 5th gear.
he's selling for 60£ including shipping from Portugal.
you can check the item in this advert:
http://faro-faro.olx.pt/carrete-da-5-mais-longo-polo-g40-polo-86c-polo-6n-iid-432515349
cheers
André