Turning off the 3d view did the trick - and I was able to render and add more populations and render these too! (screenshot attached).
It crashed when I brought back the 3d view - but not immediately, I was able to see the trees (bounding boxes) and change my viewpoint for a short while; it was when I went to the Camera tab I got the 'stopped working' error.
I will try experimenting as suggested with the preview and 3d view settings and hiding the populations - I'm currently rendering a view where the population preview was set to wireframe (if the render box appears I know it's going to work).
I think I might be saddled with the Intel graphics set-up. If I search for software online it says the best driver is installed- though yes, it might not be best for TG. I don't know how I would go about rolling it back to test an alternative. If I choose to browse for driver software on my computer the Intel one appears, and "Standard VGA graphics adapter" which sounds a bit basic!
So,.. more things to try in TG itself re the 3d previews; at the very least I suppose I could use a low res render to preview the scene.
Thank you all so much for your help on this - I was really stuck
Simon