Hi, thanks for the pointer. I did manage to find that in the end and am getting there!
So within my SitefinityWebApp project, I created a "Navigation" folder under "Mvc\Views" and created a file "NavigationView.XXX.cshtml" and copied the contents of "NavigationView.Horizontal.cshtml" in the bootstrap resource packages folder. I just changed the Brand text to prove it works. The bootstrap submenu wasn't working until I realised I needed to point the Url.WidgetContent to "ResourcePacakges/Bootstrap/assets/etc..." and now the menu is rendering to my template and the bootstrap submenu is working good.
So final thing (for now) is that the drop down menu is not rendering even though I've not excluded it yet (so there is a line the does a @Html.Action("Getview" etc. for the Dropdown view.) I'm guessing this is happening because the Dropdown view is now not relative to this template? How would I still reference it if I needed it? (which I don't, but so I know)
I also tried copying that same Dropdown.cshtml into my project folder but it's not picking that up either.
Thanks for your help!