That stuff in the ball cock

[Adrian]

Every so often the water tank starts to take an age to fill, to the extent that there is insufficient water to run a whole bath. I then have to take the ball cock apart and extract what look like bits of grit from it that are blocking the flow.

What are these things and how do they get there?

[hellymedic]

Are you in a hard water area?
Hard water deposits calcium carbonate almost everywhere.

Almost any acid will shift it.

[Adrian]

It is a hard water area and there is some limescale involved, but at the core there is something around which the limescale forms.

[fred the great]

I assume you have a normal system and the ballcock is in the header water tank. The black grit in that case is in the water supply. Sounds to me like the Water Authority has repaired a pipe some where and grit has entered their pipe and is being sent to you. In that case it should clear up naturally.

I should ring them with the problem and ask if they would help.

[rogerzilla]

I've never seen that before.  Just normal limescale (which varies in colour from white to pink to blue to green depending on where it forms - it's quite pretty inside copper pipes).