Thanks for the replies folks! Helfpul stuff. I followed up on KFX format (which I have never used until now) and that works well for me. Installed the Calibre plugin (and also it needed the Amazon Previewer installed). It works well in Calibre Convert ...
Thanks again - KFX from here on!
And yes - I was a bit muddled in my posts. I did mean epub-to-Kindle and I should have made clear that I was using an epub converted to AZW3 via Cailbre. It was this that showed the changed text selection behaviour.
Anyway - it's all good now ...