Ah! Ok I'll take a look at that. I was wrong about Lightroom, I got it mixed up with Viewnx. In fact I got a little too confused by the differences that I ended up making some notes. This is what I've found so far.

Lightroom: Reads xmp or iptc data / Writes xmp data, (duplicates iptc data for JPG but not for NEFs) adds xmp-crss fields for JPG & (NEF sidecar xmp)

Acdsee: Reads xmp data or iptc data / Writes xmp (for sidecar xmp JPG or NEF), duplicates iptc when using batch set mode for JPG

ViewNX: Reads iptc data (ignores xmp) / Writes iptc, duplicates to xmp

Capture NX2: Reads iptc data, xmp data (except categories which it reads from iptc) / Writes iptc, duplicates to xmp

This is fun ... ok I'm lying.:)
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I'll check out iptc2xmp.args. It would be nice to be able to do this all in one place so that would be a neat solution.

Thanks for your help.