Popular Routes
Got it! In the top left corner of the profile display is a tiny label "Max Y". Click on that and you get to set Max Y (or set it to Auto).
I've somehow managed to get the route profile showing a max altitude of 2000m, which isn't great for routes in coastal England. I'm pretty sure it hasn't always been this way, but I cannot see how to change it to the 400/500m I would prefer. Help!