I tried it, it gets rid of the thinner bits but not the caked on, thicker bits. I guess I need something a bit more powerful than lemon juice, or otherwise repeat the process several times to get rid of it.I hope you find it to be useful/effective.
On stovetop (or, in microwave), mix 1 cup of lemon juice and 1 tablespoon of cornstarch. Heat until mixture thickens. Will thicken further, as it cools. Once cooled, apply where needed. Leave on limescale, as long as possible. Wipe clean with a damp cloth. You may find some areas to require scrubbing.
I don't know how it got there, the toilet bowl is a place that gets cleaned regularly.