None wrote:

I changed my dishwasher the other day to be horrified at the amount of
mud underneath the old unit. I then checked under the dishwasher...
same. To see the extent of the problem I drained the attic tank, and
there was a thick brown sludgy mess at the bottom also. I wonder is it
due to a mains leak somewhere? Any help/advice on this would be great.
I'm on a mains water supply with strict regs... so I don't think this
should be happening?!

We had exactly the same problem in our house. It was certainly soil
rather than any other sort of crud. A few months later, a water leak was
discovered in the mains supply under our drive.

It was explained to us that although the water was at a fairly high
pressure, it escaped into the soil which pretty well withstood the water
pressure, but a cavity was formed and water would swirl around in that
cavity at something like mains pressure and find it's way back into the
pipe at times when the pipe pressure momentarily dipped - flushing the
loo etc. At that instant, the pressure in the cavity was equal or
greater than in the pipe and water and soil could flow into the pipe.

When the pipe was fixed the soil in the water never came back.

I've no idea if this explanation is reliable, but it sounds reasonably
plausible.