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