Thanks Raeki.
On the 3GS, the firmware update has to be signed by Apple's signature server. Soon after they release a new version, they pull the old version. Not so for the 3G?? Saurik's signature server gets around this by replaying SHSH, but if you don't have a 3.1.2 SHSH on file and Apple have pulled it, I'm not getting to that version. I needed to point your host file to the following:- 74.208.105.171 gs.apple.com which I hadn't done yet. So this iPhone is going to be LOCKED again and that tuff luck.