I figured it out. In case anyone is interested it's like this; I installed an app that is just called 'Battery'. When I open it up, it shows (among other stuff) a small patch titled 'Notifications'. By touching that patch I can toggle on or off an advice in the swipe down from the top of the screen that tells me the following information;
* what %age is left in the battery
* if it's charging, how long it will take to charge to 100%
* if the Nexus isn't charging, how many hours of use the battery has left in it.
I must have accidentally turned this function of the app off. I've turned it back on and find it useful to have the above data in the swipedown.