I quizzed my GAME buddy a bit more and it turns out that the devs themselves attended this particular promo event at Newcastle in the North East of England and actually brought a bunch of steelbooks with them which they were happy to sign for anybody who got one. My friend had two and kept an unsigned one, very kindly giving me the other so they weren't all signed at least.
Sadly this news means that my steelie isn't perhaps as scarce as i'd hoped but, I'm told, a lot of fans just wanted the regular game signed for the most part. Nevertheless an interesting addition to my PS3 steelbook collection.
