It is amazing what a difference spending a minute or so cleaning the filter in the kettle made this morning. Pouring out the water had been turning into a bit of a chore but, with the the limescale washed off, it poured out swiftly and cleanly (to the point where I almost overshot the mug!).
That probably indicates it is time to clean the inside again by boiling a vinegar and water mixture; it also serves as a illustration of how slow accretions can hamper smooth operation and a quick clean from time to time is essential to restore proper operation (make of that parable what you will!).