When you mentioned other versionsof the sword .obj, I looked back at the versions of the shorts as well, and realised the one which textured properly was a one-piece object and the failures were 2-piece - sure enough the sword has 3 parts,each named. 

Poser imports all bits of the sword as one thing ; UVMapper won't map and save the 3 parts as one thing, but it will do so for each individual part if they are seperated, selected and assigned. It hadn't actually failed, there was nothing it recognised so it saved "nothing" to the .obj.
It looks as if its ok now (with a tweak or 2)

Thanks ockham, it's dead useful to talk on the forum. So often it nudges your thoughts in the right direction.