I’d like to trial Libreoffice to see how I like it, but will it over-write Office 365?
No, it will not overwrite anything of Office 365. If you install LibreOffice, there will be a page where you can decide, whether you want to change the double-click behavior so that file name extensions like docx will open in LibreOffice. You need only be careful, that you do not check that option. Only the file extensions of the ODF formats [e.g. .odt and .ods] will be assigned to LibreOffice automatically, so that a double-click on such documents will no longer start Office 365 but will start LibreOffice.
But if you will only try it, you need not install LibreOffice. You can use a portable version [1] or make an “administrative” installation [2]. Both ways you get a folder which contains all what LibreOffice needs. Nothing is changed in your Windows registry. The only downside of this is, that you cannot start LibreOffice by double-click on a document, but you have to first start LibreOffice and then open the document via menu File > Open.
[1] Portable versions | LibreOffice - Free Office Suite - Based on OpenOffice - Compatible with Microsoft
[2] Installing several versions of LibreOffice in parallel - The Document Foundation Wiki
Thanks, that’s helpful- I’ll see what I can do with those links when I’m wide awake.