Magic.

Opened the no-show breeches pp2 side by side with the primitive Box pp2, changed all the Box references to breeches and then looked for similar parts of the file that had different parameters -all in Offsets? Carefully altered the Box values-all 0 or -0? to the ones in the breeches pp2, noted the path to the Box obj  and altered that to point at the breeches obj. 
Then remembered to copy the breeches obj to that folder (I've forgotten to actually move the target file before, took an hour to realise I was a pillock) . 
Finally saved the Box pp2 under the new name Breeches02, restarted Poser and with bated breath clicked on the tick.  

There it was...well actually there was nothing in view to start with, when I pulled the window out, there 400 feet above the ground plane was the world's biggest pair of trousers, just like a novelty hot air balloon. 
The normals were reversed so I went back to the 3D modeller and reversed them, then saved the breeches obj  back to the right poser folder. A bit of scaling and resaving and I've got my breeches prop.
Now to try turning them into conforming clothing............

EnglishBob, you are the next King of England