OK, at the risk of going all Domestic Goddess here, don't bother with all that expensive crap. White vinegar is by far the best stuff to get limescale and it's associated dirt off and it's only about 45p for a large bottle. Just douse a load of vinegar on the limescale then attack it with either a brush or some form of scourer. Repeat until the limescale is gone.
Yes it stinks to high heaven which is the vinegar mixing with the CO2 given off by the chemical reaction. The sticky Calcium Carbonate (limescale) becomes brittle Calcium Oxide which flakes away easily when you rub/brush it.
Wear your best marigolds though, otherwise the vinegar plays havoc with your fingernails 😛