Harpic Power, comes in a black bottle, is the best product I've used. Leave it on as long as possible, eg overnight, and then use a brush to remove the limescale. It really works.
Vinegar. Use the cheap distilled ALDI/LIDL stuff. Pour it around the ceramic just above the water level, leave overnight and brush next morning. I find the colour in the vinegar highlights any scale left.
Another vote for Harpic (doesn't have to be the black one, any limescale remover one does) left in for as long as you can - brought a toilet that hadn't been done in many years back to new-looking for me.
I have heard a bottle of Coca-Cola does the same job - useful if you have a septic tank and can't use the nuclear options.
Just for those of you interested in using the Coco Cola method. I completely emptied the toilet bowl this morning and poured a two litre bottle of coco cola into the bowl and left it for over 8 hours and it hasn't really made much difference so I'll be trying vinegar next and if that doesn't work I'll be getting the Harpic Power.