Good evening to you and Daniel. Thanks for your suggestion. It was very similar to what I had tried so I tried a few things.
First I tried your suggestion of putting the styles directly into my app.xaml file. Fail.
I then tried a new shell project and it worked.
I tried going back to my original shell and removing my many modules. Fail. So I then knew it was the shell. A quick 2 minute review of the two shells revealed the differences. My app.xaml + cs was not the same as the current version.
I guess over time it got changed and mine was generated from the original template build. It was only a tiny difference but enough to completely break!
For others reading the only additional thing you will need to add at the top of your app.xaml is the namespace references (with assembly)
So the app is working perfectly again. Now I shall style the app to look how I wish.
Fantastic work from you both