Hi there. Thanks for the prompt response but that isn't what I was looking for.
I wanted to find how to use Windows/Office specific fonts in other applications and install them in the Mac Font book.
I've since remembered how to do it and I'm not sure if this breaks any rules but if you have Office installed on your Mac you can find one of the apps in Finder and double-finger tap on one of them like Excel or Word. In that menu to go to Show Package Contents, go to Fonts and then choose the font you want. Open it with Font Book at which point you'll be offered the option of installing it. Job Done!
I just wanted Calibri so I could edit Microsoft originated docs with the correct font.
If anyone is reading this I hope that helps.
Thank you