I went into my Apple Store (UK, Regent Street) and managed to blag a W key from the Genius Bar. They pulled out a PB membrane from a drawer, slapped it on the counter and then asked what key I'd like. Didn't cost me a penny as I was prepared to fit it myself.
Unfortunately it was from a previous model and didn't quite fit. The catches underneath the key didn't align with my PB.
I bought another laptop for a friend. I took off that W key and swapped it with mine and they are both stuck firm now. Weird but that's my story.