Just to add a note to this, wish I'd searched here first.
I need a spare key, but after buying a key on ebay listed as 'just needs to be programmed to the car and the blank cut', I found they are indeed a one shot deal - once programmed to a car, you can't just take them and program them to a different one. It needs to be a new, virgin key to be added.
On top of this, the ebay seller (in Singapore) was a real arse about it and wouldn't even answer communications (polite ones, by the way). I only finally got my cash back after Ebay judged the dispute in my favour. He complained about having to refund and went as far as saying things bought from Ebay were 'sold as seen', effectively saying he believed he had no responsibility for the items he sells.
So I am £52 out of pocket after paying the Alfa dealer to try and code it in.
Moral? Be careful where you buy the keys from.
Make something idiot proof and someone will only go and build a better idiot...