I've been battling with limescale build-up for years. Then I saw the Mr Muscle product in Willo and bought a bottle. Here's what I did:
Pushed out as much water from the bowl as possible by repeatedly plunging the toilet brush down it.
Drained the residual water using a turkey baster (about 5 minutes).
Dried the bowl with industrial floor wipe.
Squirted a good 1/3 of the bottle all over the limescale.
Left it for 6 hours. The limescale had melted away completely, leaving a pristine white bowl as if it were brand-new.
Oh, and I put the turkey baster in the garage with a label on it "not for food use".
MM