Just an update for anyone else - I was reading the thread on GitHub for (Add support for Visual Studio 2017 templates)
And read something about the project templates - since I already have monogame 3.5 installed on the PC and use VS2015, I thought I would try copying the templates from the VS 2015 folder to the VS2017 folder: e.g.
C:\Users[username]\Documents\Visual Studio 2015\Templates\ProjectTemplates\Visual C#\MonoGame
Voila - the templates showed in the "Add New Project" window, I've managed to open up a sample monogame desktop app and it worked... So this is use full for existing installations - but as mentioned I think the issue will be on new machines.
Hope this helps others.