Not sure how to do this … If I make a high poly model then decimate and export to make uv’s in Modo. How can I get the detail from the high poly sculpt and bake as a normal map on to my decimated version once I have the uv’s done ?
That’s not quite the process. You can do your UVs before decimating. You can use UVMaster. Once you have clean UVs, you create the NMaps. Then you decimate, but pay attention! You have to keep your UVs.
that’s all 
No work around at all ? Thing is the uv’s always come out wrong in Zbrush, never found why.
Then you can do your uvs out of zb then bring your new model with uvs.
Is there something missing from the last post ?
You could also try this.
Create UVs on your Hi-res model.
Duplicate the Hi-res model. Decimate the duplicate.
Turn on the Keep UVs button when you decimate.
Take your Hi-Res and decimated model into a program like Xnormal and create your normal maps.
Hope this helps.