That metallic pan scourer you can buy that looks like thick shiny wire wool works a treat for getting off limescale from glass vases without scratching up the glass.
But when I tried to use it clean off thick limescale on the bottom of the toilet bowl it leaves a grey kind of mark on the enamel.
Any suggestions for scraping off the limescale without damaging the enamel ? I've tried liquid limescale remover and it does not do the job.
You need to find a liquid which works. You might want to mix in some detergent too (but never mix in any proprietary cleaners - just some washing up liquid, as it's normally limescale mixed with organic matter. Leave it in there overnight to give it time to act, and then the deposit should easily wipe off with a toilet brush.
Good kettle descalers work (dissolve crystals in warm water before pouring in). Furnox DS-3 works (but may leave the pan stained light blue for a few days afterwards, as the indicator dye in it is quite potent). Again, dissolve in hot water before using. Brick acid (conc Hydrochloric acid from a builder's merchant) is the strongest option (just need a tiny amount), but I wouldn't use it regularly, and make sure it's well flushed from all the pipework afterwards (otherwise it will eventually dissolve cast iron pipes and the mortar in underground pipe joints).
Big warning: Never mix different cleaners, or cleaners and bleach - some combinations quickly give off toxic gasses. Always make sure one cleaner is well flushed from the pan and the pipework after use (can damage some sewer pipework if left there) when you've finished, or when you are going to try another cleaner.