I called Justin at the TTshop today and explained my situation.
After a bit of humming and heying and looking at some pics, he told me that an OSIR GTI carbon fibre valance will NOT fit on an R32 rear spoiler. The R32 rear valance clips onto the bumper - but the GTI one wont have the clips to fit.
He thinks the GTI one is designed to be stuck ontop of the existing one - but cant confirm.
He did offer to order me an R32 spoiler in and they would try fitting the GTI valance there - so save sending it to me if it didnt fit. I would only need to pay a £15 or so restocking fee for the spoiler if I didnt need it.
Looks like I'm just going to have to wait for an Ed30 spoiler with built on valance...