None wrote:

it comes from a hot water cylinder... and having run the upstairs hot
taps, it seems to also come from there too, particularly the bath. Any
ideas?

I would go and have a look in your cold water tank and see what if
anything is floating in there for starters!

Also do you have any dead legs of old pipework that rise somewhere but
have then been capped off? This can be a nice breading ground of
stagnant water.

--
Cheers,

John.

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