I'm writing this question while connected to a friend's WiFi, with mobile data turned off. But my phone says "connected, no internet access", and there's a ! by the WiFi indicator on the bar along the top of the screen.
I think android thinking there's no internet is stopping certain apps from working, where as others don't seem to care (and work).
Why is android thinking there's no internet when there is? Can I assure it that there is internet?! I'm on 6.0.1