Thing is, if I unplug the Mesh and put the Nighthawk into modem only mode I'd still expect it to connect to the internet and show a white LED light.

I agree. It  should show an "Internet connected" LED. (Not sure about the colour.)

Someone said I needed to add the PPPoE details into the Google Mesh, sounded strange but it's the only thing I haven't tried - what do you think? I'd have thought the PPPoE was in the modem (Nighthawk)

Why strange? As @plemans and I have said that is exactly what you have to do.

Believe me, I know from experience as I have had to set up a new router a half a dozen times this week. (Testing a new device.) It always needs PPPoE details. Without it  the router cannot connect.

Perhaps that is where you are going wring. What does the manual for the H2D say?

In the case of my new router, and older routers that I have installed, it has an automated "wizard" that I can run to automatically detect the Internet settings.

Just another user with a set of boilerplate macros.