Ok ..... For anyone finding this thread with a similar problem, I have a workaround.

First I created a new project and set the style for the 'SENSUAL' with the horns. I set this new project to have the same number of bars and choruses etc as my previous project I want to change.

Next I opened my previous project, selected all the bars and copied them.

I pasted these into my new project and this played back with the new style.

I saved the new project with a different name and closed BiaB.

AS a test, I re-launched BiaB and opened my new project. Everything worked; it retained the new style with horns and played back perfectly.

Finally, out of curiosity, I tried changing the style of my new project and it all worked perfectly. I tried this a few times and was able to change the style at will.

My old project must be corrupt in some way. Now to test all my other projects and to do the same with them if needed.