Black gunk in loo and from upstairs cold taps

Tiddlywinks

On and off we get dark gunk which sticks to the sides of the loos - it comes back after a few days even though we've really scrubbed and bleached them.

Also, we have had a couple of occasions where some dark gunk has dripped from a cold tap - one not used for a while.

Do we need to get a tank flushed or treated do you think?

When I asked our regular plumber he said the black stuff from the tap might just be a sealant / coating coming through... that doesn't seem to ring true.

Any ideas welcome.

Thanks

keystone

Why don't you go up into your roofspace and see if your tank is full of black gunk? Whilst you are up there you could make sure it has a cover on it.

Cheers