The mess underneath the appliances was definitely soil, as I gave it
the 'smell test' unorthodox as that may seem... but not rust. I have
had to clean out the attic tank before because of this, but I presumed
that it was a normal thing, now I doubt that. The water has a yellow
hue to it (slight) that I have noticed, but I suppose I will ring the
water supplier tomorrow, although I'm pretty sure they won't find
anything wrong outside. The plumbing under the sink looks suspect and
the black plastic main pipe is lightly covered in soil smelling
stuff... weird. There is a strong 'earthy' odour there too.