Removing thick limescale

[HDi Dave]

White vinegar, I learnt it from how clean is your house years ago

Me too lol, and I read 101 uses for White Vinegar - natures cleaner, and to be fair it's worked everytime

I also read that, would I wouldn't use it to get creases out my shirt out go out smelling on vinegar.

I wouldn't think that was practical either

But in regards to the OP if that doesn't shift it you could always try pouring in 1 cup baking soda, then one cup hot white distilled vinegar. Let this sit for 5 minutes or so, then run hot water down the drain.