Some kind of crystals in the cistern, so loo won't flush - what to do?

[purpleturtle]

Our toilet flush has practically seized due to some kind of crystals or something (bit like iron filings?) that have grown around the shaft of the flush inside the cistern.

They seem pretty firmly fixed: I can't chip them off or scrub them off. So, anybody got any suggestions of something that might dissolve them, or otherwise persuade them to shift?

limescale remover?

Thanks. Will give that a go. I tried vinegar, which I thought would have the same effect, but to no avail. Guess we need something a bit more potent.